CHEMISTRY FORM 5 ORGANIC CHEMISTRY -CARBONYL COMPOUND

CHEMISTRY FORM 5 ORGANIC CHEMISTRY -CARBONYL COMPOUND

UNAWEZA JIPATIA NOTES ZETU KWA KUCHANGIA KIASI KIDOGO KABISA:PIGA SIMU: 07872327719

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3. Pats Papers

ORGANIC CHEMISTRY -CARBONYL COMPOUND

These are organ compounds with Carbonyl group 

as functional group

If the carbonyl compound is directly bonded to two alkyl  groups two aryl groups or one group and one aryl group, the resulting carbonyl compound known as KETONE.
I.e. General structure of kenton can be represented as (

)where R and R’ can be  alkyl or aryl group

If the carbonyl compound is directly bonded to at least one hydrogen atom, the resulting carbonyl compound is known as ALDEHYDE
I.e. General structure of aldehyde is

where R is alky group, aryl group or hydrogen atom

Both aldehyde and Ketone (carbonyl compound ) have general molecular formula 0f C_n,H_2n 0, thus aldehyde and Ketone are isomeric ( exhibit position isomerism )
Most of properties of aldehyde are the same as those of ketone. But there is some differences in their properties due to differences in their structures.

NOMENCLATURE OF CARBONYL COMPOUNDS
The parent chain in must contain carbonyl group

i.e.The parent chain is the longest continuous carbon chain with carbonyl group Numbering of carbons is done as follows

For aldelhyde the carbon in carbonyl group must be kept first position
For ketone numbering must state at the end closer in carbonyl
For ketone with more than four carbons, position of carbonyl must be indicated by using Arabic numerals

In naming aldehyde leave the suffix -e from the corresponding hydrocarbon and the end with suffix- al and naming ketone leave the suffix e from name of corresponding hydrocarbon and then end with suffix one
Other rules are the same as those of alkanes rule

Example.

Qn.Name the following organic compounds.

     

PREPARATION OF CARBONYL COMPOUNDS

a) REDUCTIVE OZONOLYSIS OF ALKENES

Alkene reacts with ozone followed by reductive hydrolysis done by

under presence of yielding carbonyl compound.

Generally:

If there is at least one hydrogen atom at each carbon with double, bond aldehyde is formed

Example:

If one carbon with double bond has at least one hydrogen atom and another carbon has no hydrogen atom , aldehyde and ketone are formed

If there is no hydrogen atom which is directly bonded to carbon with double bond, ketone is formed

b) HEATING OF CARBOXYLIC ACIDS
When carboxylic acid is heated to temperature about 300⁰ C under presence of manganese (iv) Or (MnO₂) carbonyl compound is formed heating of methanoic acid produce methanal (aldehyde)

I.e.

Memorizing

·
Heating higher members of carboxylic acids produces

Memorizing

Example

Memorizing

·         Heating methanoic acid and higher member of carboxylic acids produce aldehyde

I.e.

Memorizing

Example

c) HEATING OF CALCIUM CARBOXYLATES

When calcium carboxylates are heated to temperature of about 400

,carbonyl compounds are formed. Heating calcium methanoate produce methanal

Memorizing

 

Heating higher members of calcium carboxylates produce Ketones

Example

Heating calcium methanoate and higher member of calcium calylates produce aldehyde
i.e

Example

Note
Reactions used in preparations of carbonyl compounds in (b) and show an increase in number of carbon form n to 2n-1 in these which the same type of acid is applied so reactions are very weak which is dealing with  conversions problems which show that number of carbons has increased from n to 2n -1

Example:-

ANS.

Alternative.

(d) ACID AND MERCURY SULPHATES

Examples

 

(e) OXIDATION OF ALCOHOL

Primary alcohols give aldehyde (the oxidizing agent must be weak like

oxidation of aldehyde to carboxylic acid)

The more appropriate method of preparing aldehyde which ensures higher yield percentage of formation of aldehyde rather than carboxylic acid is heating alcohol under the presence of copper pirates catalyst.

Oxidation of secondary  alcohol yield Ketones

f) REDUCTION OF CARBOXYLIC ACID

Carboxylic acids can be reduced to aldehyde

Example

In above reaction reducing agents (Li Al ) must be limited amount so as to prevent further reduction of aldehyde alcohol.

Benzene reacts with acyl compounds yield aromatic ketone
i.e

Example

Analogously

 h) REDUCTION OF ACYL COMPOUNDS (ROSENMUND REACTION)

Acyl compounds react with hydrogen gas under the presence Palladium Catalyst poisoned with sulphur containing catalyst (mainly BaSO₄) is used to form aldehyde
I.e.

In above reaction the use of BaSO₄ (Sulphur containing) is to poison palladium catalyst so as to preview reduction of aldehyde to alcohol

I.e.

Show how would you convert

ANS

                                                                                                               

Alternatively

Alternatively

 

PHYSICIAL PROPERTIES OF CARBONYL COMPOUNDS

In comparison to alcohol carboxylic acids carbonyl and compound have lower boiling point due to limited hydrogen bonding present in them (There is weaker hydrogen binding existing between molecules of carbonyl compounds than that present in alcohol and carboxylic acid )

Among carbonyl compounds themselves boiling point increase with an increase in number of carbon atoms in the compounds Carbonyl compounds are slightly soluble in water due to their ability of forming hydrogen bonding with water molecules but are more soluble in organic solvents

CHEMICAL REACTIONS OF CARBONYL COMPOUNDS

a)      a) NUCLEOPHILIC ADDITION REACTIONS

i.  In Carbonyl group

, )Oxygen being more electronegative than carbon, Oxygen is negatively polarized and C is positively polarized is
Thus in carbonyl group is electrophilic and hence it becomes a good site for incoming nucleophile.

Yet there is π- bond in carbon of carbonyl group (functional) then carbonyl compounds are more likely to undergo addition reactions i.e. nucleophilic addition reactions.
Ability of carbonyl compound to undergo nucleophilic addition reactions depend on the amount of partial positive charge present in carbon if the carbon is more positively, polar than the compound will be more likely to undergo nucleophilic reaction and versa.
On another hand the amount of partial positive charge. Present in the carbon is determine by the strength of positive inductive effect which is exerted than the carbonyl will be less positively polarized and vice-versa

This explain why higher member of aldehyde are less reactive towards nucleophile than lower member because the strength positive inductive effect increase with an increase in length on chain of alkyl group.

The strength of positive inductive effect also explain why aldehydes are more reactive than keteones because while in positive inductive effect is exerted by one alkyl groups from two directions. In ketonepositive inductive effect s exerted by alkyl group from two direction, thus carbonyl group experience stronger positive inductive effect than in aldehyde.

Another reason of low reactivity of Ketone compaired to aldehyde is the fact that: (carbonyl group in ketone) experience large steric hindrance from the alkyl and aryl groups aldehyde there is only one alkyl group and this can also used to explain why carbonyl group with large alkyl group is less to explain why carbonyl group with large alky group is less reactive towards nuclephile ( in nuclephilic addition reaction) because steric hindrance with increase with increase in size of alkyl group.

Example

Qn. Arrange the following organic compounds according to ability of undergoing nucleophilic addition reactions.

ANS

Generally carbonyl compounds follow the following mechanism in nucleophilic addition reaction

Examples of Nucleophilic addition reaction

i)Reaction with hydrogen cyanide (HCN)
Carbonyl compounds react with HCN under presence of sodium cyanide and strong acidic medium like HCl yielding cyanic hydrin

Generally:

Example

But

1,2   —–phenyl-2-hydroxyethanone
ii)Formation of bisulphite
Carbonyl compound react with

  resulting to formation of bisulphite

Generally:-

Examples

NOTE
Thus reaction involved an introduction (addition) of NaHSO₃ is very large molecule so there is large  steric hinderancially in aromatic, ketones the reaction will not occur
e.g

iii)Reaction with alcohols
Aldehyde reacts with alcohol to form homo- acetal which is unstable but if alcohol present in excess a stable acetal is formed

Generally:-

Hemo-acetal Acetal

 

Example

Thus overall reaction can be written as

b) NEUCLEOPHILIC SUBSTITUTION REACTION

When carbonyl compounds is heated with halogenating agents like

Oxygen in carbonyl group is replaced by halogens

Example.

In above example the reaction is similar to that in alcohol difference is that HCl is not evolved ( like in alcohol) there is no

group

·     Ketones resist oxidation but they can undergo oxidation under very vigorous condition
i.e

CHEMISTRY FORM 5 ORGANIC CHEMISTRY -CARBONYL COMPOUND

c)       FORMATION OF ALCOHOL (REDUCTION )

Carbonyl compound react with reducing agents like

and to form alcohol

Generally:-

Aldehyde form primary alcohol
i.e

Example:-

Ketones from secondary alcohol

Example

e) FURTHER EXAMPLES ON REDUCTION

·         Formation of alkane.

Carbonyl compound reacts with hydroiodic acid in presence of red phosphorus at temperature of

to form alkane

BUT

NOTE

Carbonyl compound with alkyl group directly bonding carbonyl compound can react with halogen in the presence of acidic medium.

Example

 

 

NOTE:

Similary

c)     OXIDATION

Aldehyde are good reducing agents. Ie. They oxidised to carboxylic acid

Generally.

Example

Generally

Example

2) REACTION WITH HYDRAZINE
Carbonyl compound reacts with hydrazine to form hydrazone
Generally

Example

3)REACTION WITH PHENYL HYDRAZINE

Carbonyl compound reacts with phenyhydrazine to form hydrazone

Generally

 

4) REACTION WITH 2,4 –dinitrophenylhydrazine (BRADDY’S REAGENT)

Carbonyl compound react with braddy’s reagent to form yellow crystalline product(appear as yellow ppt ) of 2,4-dinitrophenylhydrazone)

The reaction is used as chemical test of presence of carbonyl group in the compound.

Example:

OTHER REACTION OF CARBONYL COMPOUNDS

i)Alde – condensation reaction
This is the reaction between carbonyl compound with hydrogen and alkane solution to form new compound both hydroxyl group and carbonyl group in the same compounds
Example:

 

When the product is passed through acidic medium like conc. 

  unsaturated carbonyl compound is formed.
i.e



b) Cannizaro reation
This is the disproportionation reaction which occur between hyde – hydrogen and alkane solution to form carbonic acid and alcohol.
Example.

CHEMICAL TESTS TO DISTINGUISH BETWEEN ALDEHYDE AND KETONE
Aldehyde and Ketone may be distingushed using the fact that aldehyde is good reducing agent ie. it can be oxidised easily while ketone cannot be oxidised
There are two solutions which are commonly used to distinguish aldehyde and ketones

i)                    Fehlings/ Benedict solutions which consist of

ii)                  Tollen’s reagents which consist of

(silver mirror test)

1)      BY USING FEHLINGS / BENEDICT SOLUTION

•          With fehlings or Benedict solution aldehyde give brick red ppt of copper (I) oxide (copper II) is reduced to copper(I)
•         Ketones being poor reducing agent give negative test i.e it does not reduce ,to and hence there is no formation of brick red ppt

 

Example:-

NOTE.
Benzaldehyde do not react with Benedict/ Fehlings solution

 

2)  BY USING TOLLEN’S REACTION( silver mirror test)
With tollen’s reagents aldehyde being reducing agent reduce

to A ie precipitate of which appear as silver mirror hence the name mirror test) Aldehyde react with tollen’s (Ammoniacal silver nitrate) form white ppt of silver which appear like mirror
Ketones being poor reducing Agent give negative mirror test

I.e

 

WHILE

 

OTHER CHEMICAL REACTION.

IODOFORM TEST.

•  This is the test for presence of terminal methyl group is directly bonded to carbonyl group by giving yellow ppt of iodoform 
•   For aldehyde only ethanal give iodoform test.

Example

Generally

NECTA 2000 PP₁  QN 14 (a)
QN.  Compound A which has an unbranched carbon chain, react methyl magnesium bromide to give after hydrolysis compound B. chromic acid oxidation of B gives C (C₅H₁₀O) or which gives crystal product with 2, 4- dinitrophenylhydrazine and a positive iodoform test.

i)Give the formula of A, B, C and equation for all reaction mentioned.

ii) Give the formula of possible Isomer of A that would give the same result as A in the above transformation (5 marks)

ANSWER

Molecular formula C₅H₁₀O confirm general molecular formula of C_nH_2nO

This C is either aldehyde or Ketone.

Formation of crystalline product with 2,4 dinitrophenyl hydrazine confirm that there is a Group in C ( it either aldehyde or ketone)

Since C give positive lodoform test then there is terminal methyl group which is directly bonded to carbonyl group in C.

A compound which gives carbonyl compound on oxidation is alcohol

This B is either primary (1°)or secondary

Carbonyl compound which give secondary

alcohol with griginard reagent ( in this case methyl magnesium bromide) are higher member of aldehyde rather than methanol which give primary alcohol .

This A is either methanol or higher member of aldehyde .

Product formed after reaction between methyl magnesium  bromine followed by hydrolysis indicate that there are 4 carbon in A

so A must be aldehyde with A – carbons and B must be secondary

alcohol with 5 carbons

Since A is unbranched

Structure formula of A

Structure formula of B is