Chemistry Chemistry As Level(Form Five) Chemistry Notes Form Five (As Level)

CHEMISTRY As LEVEL(FORM FIVE) – ORGANIC CHEMISTRY 3

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ORGANIC CHEMISTRY 3

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 Cn,H2n 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) 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)
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
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 CnH2nO
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



CARBOXYLIC ACID AND ITS DERIVATIVES
These are organic compound with carboxylic group as functional group.
They can be aliphatic or aromatic
Carboxylic group result after mixing carbonyl group and hydroxyl group.

NOMENCLATURE OF CARBOXYLIC ACIDS
Example
i) Name Propanoic acid
ii) name 3 – Pentanoic acid Or Pent -3-enoic acid
iii) Name: 4 – aminopentanoic acid


PREPARATION OF CARBOXYLIC ACID
i). OXIDATION
a) Oxidation of alcohol
Alcohol can be oxidised by strong oxidising agent like andKMnO4 to give carboxylic acid.
Where is a strong oxidising agent
EXAMPLE
If oxidising agent is weak or is in limited amount aldehyde is formed.

b/ Oxidation of carbonyl compound
Aldehyde can be oxidised easily to give carboxylic acids
i.e

Where is oxidising agent
Example

Ketones resist oxidation but under drastic (vigorous) condition it can form carboxylic acid
Example
NOTE
Methanoic acid cannot be prepared by this method (Oxidation method) . Since it can farther oxidised to and
ii) HYDROLYSIS
Acyl compound can be hydrolysed to give carboxylic acid
b/ Acidic hydrolysis of Nitrile
Generally
Example
III FORM GRIGINARD REAGENT
Carbon dioxide reacts with Grignard reagent followed by acidic hydrolysis to form carboxylic acid.

Generally:-

Example
PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS
Carboxylic acid has highest boiling point among alcohol phenol carbonyl compound or easy other hydrocarbons with comparable molecular weight due to stronger hydrogen bonding existing between molecules of carboxylic acids.
Alcohol phenol carbonyl compound or any other hydrocarbons because they come capable of making strong hydrogen bonding with water and high polarity of carboxylic group.
I.e
Carboxylic acid is capable of undergoing dimerisation when it is in the hydrocarbon solvent it or any other similar solvent without hydrogen bonding

ACIDIC BEHAVIOUR OF CARBOXYLIC ACID
Carboxylic acids have higher acid strength than alcohol and phenol.
For aliphatic carboxylic acids strength depend on:-
i/ length of carbon chain
ii/ Type of substituent in carboxylic acid
The acidic strength decrease with increase in length of carbon chain due to strong positive effect exerted by longer alkyl group.
When substituent carboxylic acid is stronger electronegative element like halogens which exert –I acidic strength is increase due to following reason:-
i/ Negative inductive effect weakens bond by reducing electron density in bond . Thus making easier to release hydrogen
proton.
ii/ Negative inductive effect stablised the carboxylate ion formed after releasing hydrogen proton by withdrawing ( partial
withdraw) – Very charged electron.
In aromatic acid the acidic strength depend on whether the substituent in benzene ring is activator or deactivator
When the substituent is activator the acidic strength with decrease and if it is deactivator the acidic strength will increase.
Example
Qn. Starting with less acidic arrange the following organic compound according to increase in acidic strength.


CHEMICAL REACTION OF CARBOXYLIC ACIDS
I. REACTION WHICH INVOLVE – COOH (CARBOXYLIC GROUP)
a/ Reaction with sodium bicarbonate ( sodium hydrogen carboxylic acid reacts with sodium bicarbonate to give solid carbonate and bubble of are formed.
Generally:-
Example

The reaction showed one of the differences between carboxylic acid on one side and alcohol and phenol on another. Ie alcohol and phenol do not react with
b) Reaction with alkali
Under this heading carboxylic acid reacts with alkali like
Generally
And
Example
i)


· Phenol reacts with and in the some way so the reaction gives of one of the similarities between alcohol and phenol.
c) Reaction with strong metals
· Carboxylic acid react with strong metals to give carboxylic salt and hydrogen gas is evolved.
· Generally
·
Where M is the strong metal
Example
i/
ii/
Alcohol and phenol also react with strong alkaline metals
II. REACTIONS WHICH REPLACE FROM CARBOXYLIC GROUP
a) HALOGENATION
Carboxylic acid react with halogenating agent like PX5, PX3 and SOX2 to form acyl halide
Example
b) Reaction with alcohol (Ester formation)
Carboxylic acid reacts with alcohol in presence of acidic medium like sulphuric acid to form ester.
Generally:-
Example
i)

BUT


CHEMICAL TEST OF CARBOXYLIC ACIDS
a/ REACTION WITH ALCOHOL
Carboxylic acid reacts with alcohol in presence of acid
The reaction is very important in distinguishing carboxylic acid from phenol is phenol do not react with alcohol verifying that carboxylic acid is more acidic than phenol
b/ REACTION WITH SODIUM BICSRBONATE
Carboxylic acid reacts with sodium bicarbonate to give effervescence of carbon dioxide gas which turn lime water milky
The reaction gives another difference between carboxylic acid and phenol as phenol do not give effervescence acidic than phenol.
c/ REACTION WITH Ferric chloride test
carboxylic acid reacts with to give Iron (III) carboxylic (alkanoate ) which appear as buff colourled compound
The reaction is important in distinguish between alcohol, phenol and carboxylic acid
  • Alcohol give no change indicating that there is no reaction between alcohol and Iron (III) chloride
  • Phenol give purple/violet coloured compound
  • Carboxylic give buff coloured compound
f/ FORMATION OF ESTER
Acyl chloride reacts with phenol in presence of to form ester
I.e.
Example
Acyl chloride also reacts with alcohol to form ester although with alcohol there is no need of NaOH(aq)


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