Unit 14: Applications of organic chemistry
Section 1: Halogenoalkanes and their reactions (nucleophilic substitution and
elimination)
Halogenoalkanes, also known as alkyl halides, are organic compounds that contain at least one
halogen atom (fluorine, chlorine, bromine, or iodine) attached to an alkyl group. These
compounds are highly reactive due to the polarity of the C-X bond, where X is a halogen atom.
The electronegativity of the halogen atom causes the bond to be polarized, with the halogen
atom carrying a partial negative charge and the carbon atom carrying a partial positive charge.
Nucleophilic substitution reactions:
In nucleophilic substitution reactions, a nucleophile attacks the carbon atom of the C-X bond,
leading to the substitution of the halogen atom with the nucleophile. The reaction proceeds via
two mechanisms: SN1 (substitution nucleophilic unimolecular) and SN2 (substitution
nucleophilic bimolecular).
SN1 mechanism:
In the SN1 mechanism, the halogenoalkane first undergoes homolysis (breaking of the C-X
bond) to form a carbocation intermediate and a halogen ion. The carbocation intermediate is
highly reactive and can be attacked by a nucleophile to form the substitution product. The rate-
determining step in this mechanism is the formation of the carbocation intermediate, and the
reaction rate depends only on the concentration of the halogenoalkane.
For example: CH3Br + H2O → CH3OH + HBr
SN2 mechanism:
In the SN2 mechanism, the nucleophile attacks the carbon atom of the C-X bond while the
halogen atom is still attached to the carbon atom. This leads to the formation of a transition
state where the carbon atom is partially bonded to both the halogen atom and the nucleophile.
The reaction rate in this mechanism depends on the concentration of both the halogenoalkane
and the nucleophile.
For example: CH3Cl + OH- → CH3OH + Cl-
Elimination reactions:
In elimination reactions, a base abstracts a proton from an adjacent carbon atom, leading to the
formation of a carbon-carbon double bond and the expulsion of a halogen ion. The reaction
proceeds via two mechanisms: E1 (elimination unimolecular) and E2 (elimination bimolecular).
E1 mechanism:
,In the E1 mechanism, the halogenoalkane undergoes homolysis to form a carbocation
intermediate and a halogen ion. The carbocation intermediate then loses a proton to a base to
form the elimination product. The reaction rate depends only on the concentration of the
halogenoalkane.
For example: CH3CH2Br + H2O → CH3CH=CH2 + HBr + H2O
E2 mechanism:
In the E2 mechanism, the base abstracts a proton from an adjacent carbon atom while the
halogen atom is still attached to the carbon atom. This leads to the formation of a transition
state where the carbon atom is partially bonded to both the halogen atom and the base. The
reaction rate in this mechanism depends on the concentration of both the halogenoalkane and
the base.
For example: CH3CH2Br + OH- → CH2=CH2 + H2O + Br-
Section 2: Alcohols and their reactions
Alcohols are organic compounds that contain a hydroxyl group (-OH) attached to a carbon
atom. Alcohols can be classified as primary, secondary, or tertiary based on the number of alkyl
groups attached to the carbon atom bearing the hydroxyl group. Alcohols are versatile
functional groups that undergo a variety of reactions, including oxidation.
Oxidation of alcohols:
Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids depending on the oxidizing
agent used and the structure of the alcohol.
Primary alcohols:
Primary alcohols can be oxidized to aldehydes or carboxylic acids depending on the strength of
the oxidizing agent used.
Alcohol to aldehyde: RCH2OH + [O] → RCHO + H2O
Alcohol to carboxylic acid: RCH2OH + [O] → RCOOH + H2O
, Secondary alcohols:
Secondary alcohols can be oxidized to ketones.
Alcohol to ketone: R2CHOH + [O] → R2C=O + H2O
Tertiary alcohols:
Tertiary alcohols cannot be oxidized by common oxidizing agents.
Section 3: Amines and their reactions
Amines are organic compounds that contain a nitrogen atom with a lone pair of electrons,
which makes them both basic and nucleophilic. Amines are classified based on the number of
alkyl or aryl groups attached to the nitrogen atom. Primary amines have one alkyl or aryl group
attached to the nitrogen, secondary amines have two, and tertiary amines have three.
1.1 Reactions of amines as bases
Amines are basic due to the lone pair of electrons on the nitrogen atom. As bases, they can
undergo protonation reactions with acids to form ammonium salts. The reaction is typically
carried out in an aqueous or polar solvent.
Example:
CH3NH2 + HCl → CH3NH3+Cl-
1.2 Reactions of amines as nucleophiles
Amines can also act as nucleophiles, attacking electrophilic species such as carbonyl
compounds, alkyl halides, and epoxides. The nucleophilic substitution reactions of amines can
be used to synthesize new carbon-nitrogen bonds.
Example:
CH3NH2 + CH3CH2I → CH3NHCH2CH3 + HI
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