CHEMISTRY FORM 5 ORGANIC CHEMISTRY-AROMATIC COMPAOUNDS (ARENES)

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-AROMATIC COMPAOUNDS (ARENES)

UNAWEZA JIPATIA NOTES ZETU KWA KUCHANGIA KIASI KIDOGO KABISA:PIGA SIMU:0787237719




ALSO READ;

  1. O’ Level Study Notes All Subjects
  2. A’ Level Study Notes All Subjects
  3. Pats Papers

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-AROMATIC COMPAOUNDS (ARENES)

AROMATIC COMPAOUNDS (ARENES)

These are organic compounds with benzene ring as functional group.

Molecular formula of benzene is C6 H6.

-It is highly unsaturated molecule but it does not undergo reaction readily and it tends to undergo substitution reaction.

STRUCTURES OF BENZENE

Structure of benzene can be expressed (shown) by using;

i. Kekule structure

ii. Resonance structure

I. KEKULE STRUCTURE (1865)

According to kekule;

-Structure of benzene is hexagonal ( It is cylic structure with six carbon atoms).

-In structure of benenze carbon-carbon double bond alternate carbon – carbon single bond.

-The structure of benzene is interconvertable.

D:\..\..\thlb\cr\tz\__i__images__i__\d18.PNG



STRENGTH OF KEKULE STUCTURE

-It gives correct molecular formula of benzene which is C6H6.

-It is true that C-H bond in benzene are all alike. (This can be seen  though x-ray diffraction).

WEAKNESS OF KEKULE STRUCTURE

-It fails to explain why benzene does not undergo addition reaction readily and it tends to undergo substitution reaction in steady.

-Through x-ray diffraction it can be seen that carbon – carbon bond are equal throughout the benzene the fact which can not be explained to by Kekule structure (According to kekule structure there is C=C  and C-C so it was expected that bond length of c=c to be shorter than that of c-c).

D:\..\..\thlb\cr\tz\__i__images__i__\e40.PNG

EXAMPLES OF ELECTROPHILIC SUBSTITUTION REACTIONS IN BENZENE

a)   (a) HALOGENATION

D:\..\..\thlb\cr\tz\__i__images__i__\O64.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\2000.jpg

MECHANISM

i. Formation of an electrophile.

D:\..\..\thlb\cr\tz\__i__images__i__\7000.jpg

ii. Formation of intermediate carbonium ion.

D:\..\..\thlb\cr\tz\__i__images__i__\8000.jpg

iii. Formation of product and regeneration of catalyst.

D:\..\..\thlb\cr\tz\__i__images__i__\3000.jpg
Thus, Overall reaction is
D:\..\..\thlb\cr\tz\__i__images__i__\4000.jpg

(b) ALKYLATION (FRIDEL CRAFT ALKYLATION)

Craft alkylation is the electrophilic substitution reaction between Benzene and haloalkane under presence of lewis acid catalyst to give alkylbenzene.

Generally;
D:\..\..\thlb\cr\tz\__i__images__i__\KK3.jpg

Example.

D:\..\..\thlb\cr\tz\__i__images__i__\5000.jpg

MECHANISM

  1.          i   Formation of an electrophile.

D:\..\..\thlb\cr\tz\__i__images__i__\60001.jpg
ii. Formation of intermediate carbonium ion.

D:\..\..\thlb\cr\tz\__i__images__i__\NN1.jpg

iii. Formation of product and regeneration of catalyst.

D:\..\..\thlb\cr\tz\__i__images__i__\O113.jpg

Hence, overall reaction.

D:\..\..\thlb\cr\tz\__i__images__i__\10000.jpg

(c) ACYLATION (FRIDEL CRAFT ACYLATION)

Fridel crafit acylation is the electrophilic substitution reaction between benzene and acyl compounds under presence of lewis acid catalyst aromatic ketone.

Generally;
D:\..\..\thlb\cr\tz\__i__images__i__\RR6.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\O310.jpg

MECHANISM

i.Formation of an electrophile.

D:\..\..\thlb\cr\tz\__i__images__i__\O410.jpg

ii. Formation of intermediate carbonium ion.

D:\..\..\thlb\cr\tz\__i__images__i__\ww4.jpg

iii. Formation of product and regeneration of catalyst.

D:\..\..\thlb\cr\tz\__i__images__i__\xx12.jpg

iv.

Thus, overall reaction is

D:\..\..\thlb\cr\tz\__i__images__i__\12000.jpg



(d) CUMENE FORMATION

Bnzene react with propene under presence of acid medium to give isopropyl benzene (cumene)

D:\..\..\thlb\cr\tz\__i__images__i__\16000.jpg

MECHANISM

i.Formation of an electrophile.

D:\..\..\thlb\cr\tz\__i__images__i__\O510.jpg

ii. Formation of intermediate carbonium ion.
D:\..\..\thlb\cr\tz\__i__images__i__\aaa8.jpg

iii. Formations of product and regeneration of catalyst.

D:\..\..\thlb\cr\tz\__i__images__i__\bbb8.jpg

Thus, overall reaction is
D:\..\..\thlb\cr\tz\__i__images__i__\ccc8.jpg

(e) NITRATION

Benzene react with Nitric acid under presence of sulphuric acid yielding nitrobenzene.

i.e
D:\..\..\thlb\cr\tz\__i__images__i__\ddd7.jpg

MECHANISM

i. Formation of an electrophile.

D:\..\..\thlb\cr\tz\__i__images__i__\eee7.jpg

ii. Formation of intermediate carbonium ion.
D:\..\..\thlb\cr\tz\__i__images__i__\fff5.jpg
iii. Formation of product and generation of catalyst.
D:\..\..\thlb\cr\tz\__i__images__i__\hhh3.jpg

Hence, overall reaction is

D:\..\..\thlb\cr\tz\__i__images__i__\iii3.jpg

Benzene react with sulphur trioxide (or concentrated sulphuric acid) to give sulphobenzene (Benzene sulphoric acid).

D:\..\..\thlb\cr\tz\__i__images__i__\jjj3.jpg

MECHANISM

i. Formation of an electrophile.
D:\..\..\thlb\cr\tz\__i__images__i__\kkk4.jpg
ii. Formation of intermediate carbonium ion.
D:\..\..\thlb\cr\tz\__i__images__i__\lll3.jpg

iii. Formation of product.
D:\..\..\thlb\cr\tz\__i__images__i__\mmm3.jpg

Thus, overall reaction is

D:\..\..\thlb\cr\tz\__i__images__i__\nnn1.jpg

Also.
D:\..\..\thlb\cr\tz\__i__images__i__\ooo2.jpg

Above reaction sulphuric acid Itself is good lewis acid (There is need of another lewis acid catalyst).



DIRECT EFFECT IN MONOSUBSTITUETED BENZENE

ACTIVATOR AND DEACTIVATOR

  •   Reactivity of benzene towards electrophile (in eletrophilic substitution reaction of benzene) depend on the electrons density in benzene ring.
  •        If the electron density is high then benzene will be more reactive towards electrophile and if it is low than the benzene will be less reactive toward an electrophile.
  •        When substitutients in benzene increase electron density in benzene ring, then the substituents in said to increase reactivity of benzene towards an electrophile.
  •        So any factor which affect the electron density in benzene ring is said to affect reactivity of benzene towards an electrophile.
  •        When substituents in benzene increase electron density in benzene ring, Then the substituents is said to increase reactivity of benzene towards an electrophile. i.e it is said to activate electrophilic substitution reaction of benzene and hence the substituent is known a ACTIVATOR.

·         On other hand if the substituents decrease electron density in benzene ring, then the substituents is said to decrease reactivity of benzene towards an electrophile. i.e It said to deactivate electrophilic substitution reaction of benzene and hence the substituent is known as DEACTIVATOR.

Qn. How we can recognize the substituents is activator or Deactivator?

ANS

Before studying recognisation of activators and deactivators. It is better to study first effect which cause activation and deactivation in benzene.

There are two effect which cause activation in benzene.

i. Positive Inductive effect (+I).

ii. Positive mesomeric effect (+M).

i. POSITIVE INDUCTIVE EFFECT (+I)
This is the effect which arise in the organic compounds as a result of partial movement of electron pair towards the functional group. (In this case benzene ring).

ii. POSITIVE MESOMERIC EFFECT (+M)

This is the effect which arise in the organic compounds as a result of total movement of an electron pair towards the functional group ( in case benzene ring) and move back again to its original position within the same molecule. Thus +M to activation in benzene substituents which cause +M (in benzene) are those with atoms possessing pair or negatively charged atom and It self directly bonded to another atom by sigma (δ)bond.

Example

OH, NH2, RO, X.

Other hand there are two effects which cause deactivation in benzene.

i. Negative Inductive effect (-I).

ii. Negative mesomeric effect (-M).

 

i. NEGATIVE INDUCTIVE EFFECT (I)

This is the effect which arise in organic compound as result of partial withdraw of an electron pair from functional group. (in this case benzene ring).

Inductive effect do deactivate of the benzene by partial withdraw of electron pair from benzene ring.

Substituent which cause (-I) are strong electronegative atom or electron attracting radicals.

Examples.    OH, X,  etc.

ii. NEGATIVE MESOMERIC EFFECT (-M)

This is the effect which arise in organic compounds as a results of partial withdraw of an electron pair from functional group (in this case benzene ring) and then moving back again to the original position within the same molecule.

  •  So -M do deactivation in benzene by withdraw of an electron pair from benzene ring.
  •         Substituents which cause –M are those with atoms possessing pair or negatively charged electron and itself is bonded to atom by π-bond.

 

Example:
D:\..\..\thlb\cr\tz\__i__images__i__\O63.jpg

·    There is the case where there is competition between mesomeric effect and Inductive effect. i.e the same substituent cause negative inductive and positive mesomeric effect (+M).

·     When this occur in most cases mesomeric effects tends to outweighs Inductive effects i.e when the same species cause –I and then the effect at which will be considered is +M and these will be ACTIVATOR (Not deactivator).

  • Halogens are exceptional of above explanations i.e In halogens Inductive effects tends to outweighs mesomeric effects why?

REASONS

  •        Halogens are strongest electronegative element among all substituent of benzene as result of their smallest atomic size. This make halogens to exert strongest negative inductive effect.
  •        On other hand Halogens have maximum number of lone pair electron, thus making less available in participation of mesomerism thus make Halogens to exert weakest mesomeric effect among all substituents.
  •        So while Halogens exert strongest negative Inductive effect it also weakest effect (-M) hence in halogens Inductive effect weighs mesomeric effect.
  •        Generally we can conclude that all substituents which cause positive inductive effect and those which cause positive mesomeric exceptional of halogens are ACTIVATOR. And all substituents which cause negative mesomeric effect with addition of Halogens (which –I ) are DEACTIVATOR.




DIRECTING EFFECT

Carbons in benzene with only one substituent group can be formed as follow;

D:\..\..\thlb\cr\tz\__i__images__i__\2001.jpg

The subustituent is activated, then it tends to direct incoming electrophile substituent at Ortho and para position i.e All activators are Ortho – para directors.

This can be explained considering;

i.Position of carbonium ion.

ii.Stability of intermediate carbonium ion.

I.  POSITION OF CARBONIUM ION

Understand this consider mesomerism of phenol in which OH activator is directly attached to benzene ring.

D:\..\..\thlb\cr\tz\__i__images__i__\O73.jpg

Above mesomerism (+M). It can be seen that despite the fact OH (activator ) increase electron density through out the benzene ortho, para positions are more effected and hence ortho and para, carbons become better site for incoming electrophile.

II.  STABILITY OF INTERMEDIATE CARBONIUM ION

1st CASE
Incoming electrophile attach at ortho position. Consider electrophilic substitution reaction in aniline

D:\..\..\thlb\cr\tz\__i__images__i__\2226.jpg

Mecomerism it is clearly understood that intermediate carbonium ion is stabilised by lone pair electrons in nitrogen of amino group (-NH2) and hence it is more stable

2nd CASE

If incoming electrophile attaches (substitute) at meta position consider the same reaction in aniline.

D:\..\..\thlb\cr\tz\__i__images__i__\4447.jpg

  • In this case intermediate carbonium is not stabilised by lone electrons of nitrogen in amino group and hence it is less stable.

3rd CASE

If incoming electrophile substituents are at para position.

. Consider the same reaction in aniline.
D:\..\..\thlb\cr\tz\__i__images__i__\5556.jpg

·         . In this case carbonium ion is stabilised by lone pair of nitrogen in group amino hence it is more stable.



CONCLUSION

Since carbonium ions formed in 1st and 3rd case are more stable than that formed in 2nd case. Ortho and para positions are prefered sites for incoming electrophile.

NOTE:

  •         Alkyl group act as ortho-para directon by doing partial neutralization of positive charge formed on the adjustment carbon

(The partial neutralization is done by positive inductive effect exerted by alkyl groups). Hence ortho – para directing of alkyl groups   is simply explained by considering stability intermediate carbonium ion like in (ii) above.
i.e.
D:\..\..\thlb\cr\tz\__i__images__i__\6665.jpg

Are ortho – para directors due to stability intermediate carbonium ion.  This is simply because despite the fact that lone pairs in halogens have not good participation in mesomerism for reason which has explained, but in presence of positive charge on adjacent carbon lone pair electrons participate in neutralizing positive charge on the carbon.
i.e.
D:\..\..\thlb\cr\tz\__i__images__i__\7772.jpg

 

  • Among the two products (ortho product and para product in most cases para product is major product why?

Reason

Due to steric hinderance exerted by the substituent originally present in benzene, ortho carbons which are closer to the substituents experience the effect strongly and hence incoming electrophile is more favoured to substitute at para carbon which is far from the substituent.

But if the substituent in halogen, ortho product become major product why?

Reason

Halogens like Cl have very small atomic size, Thus they exert very small steric hinderance thus make incoming electrophile to substitute first at ortho carbons (for every two ortho) carbons there is only one para carbon).

Deactivators with exceptional of halogens directs incoming eletrophile at meta position i.e. Deactivator (with exceptional halogens) are meta directors.

This can be explained by considering

i. Position of carbonium ion

ii. Stability of intermediate carbonium ion.

 

I. POSITION OF CARBONIUM ION

  •         To understand this consider mesomerism (-M) in benzoic acid

D:\..\..\thlb\cr\tz\__i__images__i__\9991.jpg

From the above shown mesomerism it can be seen that despite the fact that carboxylic group (-COOH) deactivate the whole benzene ring ortho and para positions are more effected and hence meta carbon somehow become pereferd position for incoming eletrophile.



II.  STABILITY OF INTERMEDIATE CARBONIUM ION

  •         Consider the electrophilic substitution reactions in benzoic acid.

1st CASE

If incoming electrophile substitute of ortho position.

i.e.
D:\..\..\thlb\cr\tz\__i__images__i__\10111.jpg

  •   Intermediate carbonium is not stable as result of very large repulsion force between closer positively charged ions in adjacent carbons.

2nd CASE

If incoming eletrophile substitute of meta position.

D:\..\..\thlb\cr\tz\__i__images__i__\400.jpg

  •         Carbonium ion formed in this case is somehow more stable as a result of comparable small repulsion force between position charged carbons which are not adjacent.

 

3rd CASE

If incoming electrophilic substitutes at para position. Also Intermediate carbonium ion formed  is not stable as a result of very large repulsion force between closer positively charged ions in adjacent carbons.
i.e.

D:\..\..\thlb\cr\tz\__i__images__i__\qq2.jpg
CONCLUSION
Intermediate carbonium ion formed in second case is more stable than in 1st case and 3rd case and hence meta position is better site for incoming electrophile.

SUMMARY ON DIRECTING EFFECT
D:\..\..\thlb\cr\tz\__i__images__i__\halogens.jpg


SOLVED PROBLEMS

QN 1. Arrange the following compounds in order of reactivity towards.

i.  Nucleophile.
D:\..\..\thlb\cr\tz\__i__images__i__\O83.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\CCC9.jpg

Qn. 2. Explain why alkylation of nitrobenzene is much slaver that of methy l benzene?

ANS
Alykylation in given compounds is electrophile substitution reaction so presence of nitro group which is an electron withdrawing (deactivator) in nitrobenzene deactivate. Its reaction towards electrophile while presence of methyl group which is electron receptor group (activator) in methyl benzene activate its reaction towards electrophile and hence alkylation of nitrobenzene become less than that of methyl benzene.



Qn. 03. Complete the following organic reactions.
D:\..\..\thlb\cr\tz\__i__images__i__\alcl3.jpg
D:\..\..\thlb\cr\tz\__i__images__i__\Y9.jpg

Qn. 04. NECTA 1994
Write structural formula of main substitutional product in the following organic reactions.

D:\..\..\thlb\cr\tz\__i__images__i__\Oo4.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\uu1.jpg
D:\..\..\thlb\cr\tz\__i__images__i__\vv4.jpg
D:\..\..\thlb\cr\tz\__i__images__i__\CLCH.jpg

Qn 05. NECTA 1993
Which substituent entered first in the following organic compounds giving reasons.
D:\..\..\thlb\cr\tz\__i__images__i__\xx13.jpg D:\..\..\thlb\cr\tz\__i__images__i__\yy6.jpg       D:\..\..\thlb\cr\tz\__i__images__i__\zz4.jpg       D:\..\..\thlb\cr\tz\__i__images__i__\500.jpg

ANS
i.   Either of the two substituents entered first.
Reason
In given compound OH and CH3 are para related and OH and CH3 are ortho –para directors forming para product as  a major product and hence either of the two entered first so as to direct incoming substituent at para position.



ii.  NO2 entered first
Reason
In given compound CH3 and NO2 are meta related so being meta director it must be entered first so as to direct the incoming CH3 group at meta position.

iii.Cl  entered first
Reason
In given compound OH and Cl are ortho related and OH and Cl are orth-para directors, OH forming product a major product (as result of its large steric hinderance while Cl form ortho product as major product (as result low steric hinderance ) and Cl must be entered first so direct at ortho position.

Reason
In given compound CH3 and COOH are para related, CH3 being ortho –para product forming para product as major product must be entered first so as to direct incoming – COOH at para position.

Qn 6.
Show how the following conversions can be achieved.
D:\..\..\thlb\cr\tz\__i__images__i__\aaaa4.jpg

ANS
D:\..\..\thlb\cr\tz\__i__images__i__\cccc5.jpg
D:\..\..\thlb\cr\tz\__i__images__i__\TY5.jpg
FURTHER CHEMICAL REACTIONS OF BENZENE
Apart from electrophilic substitution reactions benzene can undergo the following reactions.

i. ADDITION REACTIONS

  •  Under vigorous condition benzene can undergo addition reaction.

eg.
(a)    HYDROGENATION
Benzene can react with hydrogen under presence of nickel or platinum catalyst yielding cyclohexane.
i.e.
D:\..\..\thlb\cr\tz\__i__images__i__\EEEE5.jpg

(b)  CHLORINATION

  •   Under presence of U.V a very high temperature benzene react with chlorine to give 1,2,3,4,5,6-hexachlorocyclohexane.

D:\..\..\thlb\cr\tz\__i__images__i__\hy.jpg





TOLUENE (METHYL BENZENE)

  •  Toluene is the aromatic compound which is formed when one halogen atom of benzene is replaced by methyl group.

i.e.  Structure of toluene is ;-

D:\..\..\thlb\cr\tz\__i__images__i__\gggg4.jpg

PREPARATION OF TOLUENE

(a)  METHYLATION OF BENZENE

Generally;
D:\..\..\thlb\cr\tz\__i__images__i__\hhhh3.jpg

Example:

D:\..\..\thlb\cr\tz\__i__images__i__\iiii2.jpg

( b)  Reaction between halobenzene and halomethane under;
Presence of sodium and dry ether.

Generally.
D:\..\..\thlb\cr\tz\__i__images__i__\llll2.jpg

Example;
D:\..\..\thlb\cr\tz\__i__images__i__\kkkk6.jpg

CHEMISTRY FORM 5 ORGANIC CHEMISTRY-AROMATIC COMPAOUNDS (ARENES)

PHYSICAL PROPERTICES OF TOLUENE

  • It is more denser than water.
  • It is solube in non-polar solvents like organic solvent (Toluene itself is good organic solvent)
  • It melts at temperature of -950C and boils at 1110C
  • Its vapour density is large than that of the air
  • It is colourless liquid at room temperature.

In most cases toluene is used as organic solvent instead of benzene because it is less toxic.



CHEMICAL REACTION OF TOLUENE

Commonly toluene undergo the following chemical reactions
i.  Side chain chemical reactions
ii.Electrophilic substitutions in benzene ring

I.  SIDE CHAIN CHEMICAL REACTIONS
Under this heading toluene undergo the following
a)Oxidation
b)Free radical substitution reactions.

A) OXIDATION
With milder oxidizing like MnO2 agent benzaldehyde is med.

i.e

D:\..\..\thlb\cr\tz\__i__images__i__\yui.jpg

But with strong oxidizing agent like kmno4 and K2Cr2O7 ie acid is formed

eg.
D:\..\..\thlb\cr\tz\__i__images__i__\O58.jpg

B) FREE RADICAL SUBSTITUTION REACTION
·         With halogens under presence of U.V or very high temperature tends to undergo side chain radical substitution reactions.

D:\..\..\thlb\cr\tz\__i__images__i__\cl.jpg

II.  ELECTROPHILIC SUBSTITUTIONS IN BENZENE RING

Consider this heading toluene undergo similar chemical reaction as those of benzene. The only difference is that methyl group in toluene act as  ortho director forming para product as major product



Example of electrophilic substitution reactions of Toluene

i. HALOGENATION

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\halogenation_1.png
Example
D:\..\..\thlb\cr\tz\__i__images__i__\ji1.jpg
ii. ALKYLATION

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\alkylation_1.png
Example

D:\..\..\thlb\cr\tz\__i__images__i__\jk3.jpg
iii.  ACYLATION

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\ps1.jpg
D:\..\..\thlb\cr\tz\__i__images__i__\tyk.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\O92.jpg



SOME STRUCTURE OF AROMATIC COMPOUNDS $ THEIR COMMON NAME

D:\..\..\thlb\cr\tz\__i__images__i__\stucture_of_aromatic_compound_11.png
D:\..\..\thlb\cr\tz\__i__images__i__\stucture_of_aromatic_compound_21.png



II.  SECONDARY (20) HALOALKANE
These are haloalkanes where by a carbon with halogen is directly bonded to two alkyl groups
Thus for haloalkane with only one halogen the carbon with halogen is also directly bonded to only one hydrogen atom.

D:\..\..\thlb\cr\tz\__i__images__i__\secondary_haloalkane1.png

III. TERTIARY (3O) HALOALKANES

These are haloalkanes where by a carbon containing halogen directly bonded to three alkyl groups.
Thus in tertiary haloalkanes there is no hydrogen atom which is directly bonded to carbon with halogen.

D:\..\..\thlb\cr\tz\__i__images__i__\tertiary_haloalkane2.png

Haloalkanes are named by naming halogen as substituent of alkanes.

      PREPARATIONS OF HALOALKANES

a). FREE RADICAL SUBSTITUTION REACTION OF ALKANES

Generally.

D:\..\..\thlb\cr\tz\__i__images__i__\free_radical_substution_reaction_of_alkanes.png
Example.

D:\..\..\thlb\cr\tz\__i__images__i__\free_radical_substution_reaction_of_alkanes_2.png
In above reaction chlorine must be present in limited amount so as to prevent further chlorination of the product otherwise

D:\..\..\thlb\cr\tz\__i__images__i__\halogenation_of_alkanes.png

b). HALOGENATION OF ALKANES

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\halogenation_of_alkanes_21.png

Example

D:\..\..\thlb\cr\tz\__i__images__i__\halogenation_of_alkanes_example.png





C). HALOGENATION OF ALCOHOL

1.  By using hydrogen halide
Alcohol reacts with hydrogen halide to give haloalkanes

Example
D:\..\..\thlb\cr\tz\__i__images__i__\JOSSE.jpg
2.  By using phosphorous pentahallide.

Generally.
D:\..\..\thlb\cr\tz\__i__images__i__\TERE.jpg

4. Reaction by using thionylchloride
D:\..\..\thlb\cr\tz\__i__images__i__\keelvn.jpg

PHYSICAL PROPERTIES OF HALOALKANES
Melting and boiling point of haloalkane  are greater than those of corresponding members of alkanes(alkane whose moleculer mass do not differ with those of haloalkanes) as a result of highly polarity of C-X bond
eg  Cδ+ —  Clδ-

Haloalkanes are soluble in organic solvent (haloalkanes themselves are good organic solvent i.e they tend to form miscible with another organic solvent) but are almost insoluble in water .




CHEMICAL REACTIONS OF HALOALKANES.
Haloalkanes undergo the following chemical reactions

a) Nucleophilic reaction
b) Elimination reaction
c) Reaction which leads to formation of Grignard reagent
d) Wurtz/coupling/fillings reactions
e) Reduction


A) NUCLEOPHILIC SUBSTITUTION REACTION

Nucleophilic substitution reaction in Haloalkane undergo two types of mechanism namely:

i) SN mechanism
ii) SN2 mechanism

i. SN. MECHANISM

This is nucleophilic substitution reaction where by there is only one molecule which is involved in rate determining
Definition of Rate dertermining step
This is the lowest step in reaction mechanism

SN. Mechanism is more common in tertiary haloalkanes as a result of high stability brought by strong positive active effect exerted by three alkyl groups
Illustration.

D:\..\..\thlb\cr\tz\__i__images__i__\SN_MECHANISM_illustration.png

        ii. SN2 MECHANISM
This is the Nucleophilic substitution reaction mechanism which by there are two molecules in rate determining step.
It is more common in primary haloalkanes (also in secondary haloalkanes)

D:\..\..\thlb\cr\tz\__i__images__i__\SN2_MECHANISM_illustration.png

Generally reactivity of haloalkanes towards nucleophile follow the following

D:\..\..\thlb\cr\tz\__i__images__i__\SN2_MECHANISM_illustration2.png




Haloalkanes

i. Formation of alcohol
Haloalkanes reacts with alkaline solution like NaOH(aq) yielding alcohol

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\rita.jpg

ii.  Formation of amines.
Haloalkanes react with ammonia yielding amines

Generally.

D:\..\..\thlb\cr\tz\__i__images__i__\formation_of_amines.png


iii. Formation of nitroalkanes

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\as2.jpg

Where:
M is g, Na, k e.t.c

Example

D:\..\..\thlb\cr\tz\__i__images__i__\formation_of_nitroalkanes_example.png


iv. Formation of Ester
D:\..\..\thlb\cr\tz\__i__images__i__\gh4.jpg

V. Formation of Ether

Generally.

D:\..\..\thlb\cr\tz\__i__images__i__\uo.jpg

vi. Formation of Nitrile

Generally
D:\..\..\thlb\cr\tz\__i__images__i__\formation_of_nitrile.png

Nitrile is used to synthesize various organic compound
D:\..\..\thlb\cr\tz\__i__images__i__\tyu2.jpg
So the reaction (formation of nitrile) is very important in dealing with conversion problems which show that number of carbons have increase by one.



Example
Show how the following conversion can be
D:\..\..\thlb\cr\tz\__i__images__i__\STK.jpg
ANS.
D:\..\..\thlb\cr\tz\__i__images__i__\formation_of_nitrile_4.png

Replacement by another halogen

  • Halogen in haloalkane can be replaced by another halogen which is more nucleophilic.
  • The strength of nucleophilic character of halogen follow the following order.

D:\..\..\thlb\cr\tz\__i__images__i__\halogen.png

B). ELIMINATION REACTIONS.
Elimination and nucleophilic substitution reactions in haloalkane are competitive reactions.
Conditions which favour elimination reactions are

– The reaction should under taken in non-aqueous solution (in this case the reaction is undertaken inpresence of alcohol)

– Presence of strong and concentrated base like conc. KOH

– The reaction should be undertaken at hight temperature

– Tertiary haloalkane

Formation of major product in elimination reaction (if there is ability of forming more than one product ) is governed by saytzeff’s rule which. States that. “The alkane with great number of alkyl group is more stable.
– So according to saytzeff’s rule if there is possibility of forming more than one elimination product, the major product is the alkene with greater number of alkyl groups

·  Elimination product of

D:\..\..\thlb\cr\tz\__i__images__i__\TR42.jpg is ether

i.  CH3CH=CHCH3 or
ii. CH3CH2CH=CH2, so according to saytzeff’s rule (i)  is Major PRODUCT

·There are two types of mechanism of elimination reaction in haloalkane namely
a)  E1 mechanism
b)  E2 mechanism

A. E1 MECHANISM
·This is mechanism of elimination reaction where by there is only one molecule which is involved in rate determining step
– It is more common in tertiary haloalkane as result of high stability of intermediate carbonium ion which is brought by very strong positive inductive effect from three alkyl groups.

D:\..\..\thlb\cr\tz\__i__images__i__\SU1.jpg

B. E2  MECHANISM
·This is the elimination reaction mechanism where by there are two molecules which are involved in rate determining step.
– It is more common in primary haloalkane (also in secondary haloalkane) due to instability of intermediate carbonium ion which could be formed if it undergo mechanism as a result of weak +I exerted by one alkyl group.

D:\..\..\thlb\cr\tz\__i__images__i__\E2_MECHANISM.png

Examples of elimination reaction in haloalkanes are

D:\..\..\thlb\cr\tz\__i__images__i__\R5.jpg

NOTE:
Possibility of haloalkanes to undergo elimination reactions follow the following trend
Tertiary haloalkanes>Secondary haloalkanes > Primary haloalkanes




C.   GRIGINARD REAGENT FORMATION

·  Grignard reagent is alkylmagnesium halide (RMgx)
·  Haloalkane react with magnesium under presence of dry ether yielding Grignard reagent.

Grignard reagent.
D:\..\..\thlb\cr\tz\__i__images__i__\bos.jpg

D.  WURTZ REACTION.

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\WURTZ_REACTION.png

E. REDUCTION.
·  With hydrogen gas under presence of nickel or plantinum catalystal heat alkananes formed from haloalkanes.

Generally

D:\..\..\thlb\cr\tz\__i__images__i__\O102.jpg

HALOARENE

This is the halohygrocarbon which is formed when atleast one hydrogenation of benzene is replaced by halogen.
– The simplest one is formed when only one hydrogen atom of benzene is replace by halogen.

i.e
D:\..\..\thlb\cr\tz\__i__images__i__\HALOARENE.png

Chemical reactions of haloarenes include.
i.   Nucleophilic substitution reaction
ii.   Grignard reagent formation
iii.  Wurtz/Fitting/coupling reaction.
iv.  Reduction
v.   Electrophilic substitution reaction

I. NUCLEOPHILIC SUBSTITUTION REACTION

D:\..\..\thlb\cr\tz\__i__images__i__\TI1.jpg




II. GRIGINARD REAGENT FORMATION

D:\..\..\thlb\cr\tz\__i__images__i__\fy2.jpg


III. WURTZ REACTION

D:\..\..\thlb\cr\tz\__i__images__i__\na1.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\nna2.jpg

IV.  REDUCTION

D:\..\..\thlb\cr\tz\__i__images__i__\d39.jpg

NOTE:

When hydrogen present in excess, cyclohexane if formed.




V.  ELECTROPHILIC SUBSTITUTION REACTIONS

Under this heading haloarene undergo similar reactions as those of benzene, the only difference is that halogen present in haloarene directs incoming electrophile at ortho and para position forming ortho product as major product.

D:\..\..\thlb\cr\tz\__i__images__i__\M27.jpg

Qn:

Give chemical tests to distinguish each of the following pairs of organic compounds.

D:\..\..\thlb\cr\tz\__i__images__i__\O114.jpg

ANS.

D:\..\..\thlb\cr\tz\__i__images__i__\ELECTROPHILIC_SUBSTUTION_REACTION_ANS.png




With  NaOH(aq)  at  room temperature followed by  addition of  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image136.png give white ppt  of  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image137.png does not.

i.e
D:\..\..\thlb\cr\tz\Carbon compound 1_files\image138.png

Then,

D:\..\..\thlb\cr\tz\Carbon compound 1_files\image139.png

  While
         CH3CH=CH2 + NaOH(aq)  → No reaction

iv) With  NaOH(aq)  at room temperature and  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image141.png   gives yellow ppt of  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image142.png   while  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image143.png   gives white  ppt of D:\..\..\thlb\cr\tz\Carbon compound 1_files\image144.png

D:\..\..\thlb\cr\tz\__i__images__i__\O124.jpg

v) With  NaOH(aq)  at room  temperature followed by addition of  AgNO3(aq)

D:\..\..\thlb\cr\tz\__i__images__i__\O133.jpg

Ans

With  NaOH(aq)  at room temperature followed by addition

D:\..\..\thlb\cr\tz\Carbon compound 1_files\image164.png   give white ppt of  D:\..\..\thlb\cr\tz\Carbon compound 1_files\image144.png   while D:\..\..\thlb\cr\tz\Carbon compound 1_files\image165.png does not

D:\..\..\thlb\cr\tz\__i__images__i__\O144.jpg

D:\..\..\thlb\cr\tz\__i__images__i__\O57.jpg

While
    CH3CH=CH2 + NaOH(aq) → No reaction



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