A chemical reaction occurs as follows : CH3CH2CH2Br + aq. KOH B + KBr (A) a) Identify the compound 'B' in the above reaction. [1] b) Which substitution reaction mechanism occurs in the above reaction and why ? [2] c) What is the product formed when alc. KOH solution is used instead of aq. KOH in the above reaction ? [1] d) How can you obtain nitropropane from compound 'A' ? [1]

(A) Identification of ‘B’ B = 1‑propanol CH3CH2CH2OH The bromine atom of 1‑bromopropane ( A ) is replaced by the hydroxyl group from aqueous KOH. (B) Substitution mechanism Feature SN1 SN2 Rate law First‑order (depends on substrate) Second‑order (depends on substrate + nucleophile) Carbocation formation Required Not formed Stereochemistry Racemisation Inversion of configuration Favoured substrate Tertiary secondary primary Primary secondary tertiary Leaving group ability Good Good Nucleophile strength Weak Strong Mechanism: SN2. Reason: The substrate is a primary alkyl halide; a strong nucleophile ( OH^ ) attacks the carbon bearing bromine from the backside, leading to a concerted displacement with inversion of configuration. (C) Product with alcoholic KOH When alc. KOH is used, elimination predominates (E2) giving an alkene: Product = propene CH3CH=CH2 (D) Synthesis of nitropropane from ‘A’ 1‑bromopropane can be converted to 1‑nitropropane ( CH3CH2CH2NO2 ) by nucleophilic substitution with a nitrite ion: CH3CH2CH2Br + NaNO2 CH3CH2CH2NO2 + NaBr Use aqueous NaNO₂ (or KNO₂) in DMF at 0 °C → 25 °C. The reaction proceeds via an SN2 pathway because the substrate is primary and the nitrite ion is a good nucleophile.

A chemical reaction occurs as follows : CH3CH2CH2Br + aq. KOH B + KBr (A) a) Identify… – Chemistry NEB 2082 Solution

A chemical reaction occurs as follows : CH3CH2CH2Br + aq. KOH B + KBr (A) a) Identify the compound 'B' in the above reaction. [1] b) Which substitution reaction mechanism occurs in the above…

A chemical reaction occurs as follows : $CH X_{3} CH X_{2} CH X_{2} Br + aq \cdot KOH B + KBr$ (A)

a) Identify the compound 'B' in the above reaction. [1]
b) Which substitution reaction mechanism occurs in the above reaction and why ? [2]
c) What is the product formed when alc. KOH solution is used instead of aq. KOH in the above reaction ? [1]
d) How can you obtain nitropropane from compound 'A' ? [1]

(A) Identification of ‘B’

B = 1‑propanol $CH X_{3} CH X_{2} CH X_{2} OH$

The bromine atom of 1‑bromopropane ( $A$ ) is replaced by the hydroxyl group from aqueous KOH.

(B) Substitution mechanism

Feature	SN1	SN2
Rate law	First‑order (depends on substrate)	Second‑order (depends on substrate + nucleophile)
Carbocation formation	Required	Not formed
Stereochemistry	Racemisation	Inversion of configuration
Favoured substrate	Tertiary > secondary > primary	Primary > secondary > tertiary
Leaving group ability	Good	Good
Nucleophile strength	Weak	Strong

OH^-SN₂ mechanism (backside attack)propan-1-olpropan-1-ol+Br^-

SN2 mechanism: nucleophile (OH⁻) attacks carbon from the opposite side of the leaving group (Br⁻)

Mechanism: SN2.
Reason: The substrate is a primary alkyl halide; a strong nucleophile ( $OH X^{-}$ ) attacks the carbon bearing bromine from the backside, leading to a concerted displacement with inversion of configuration.

(C) Product with alcoholic KOH

When alc. KOH is used, elimination predominates (E2) giving an alkene:

Product = propene $CH X_{3} CH = CH X_{2}$

PropenePropene

Structure of Propene

(D) Synthesis of nitropropane from ‘A’

1‑bromopropane can be converted to 1‑nitropropane ( $CH X_{3} CH X_{2} CH X_{2} NO X_{2}$ ) by nucleophilic substitution with a nitrite ion:

$CH X_{3} CH X_{2} CH X_{2} Br + NaNO X_{2} CH X_{3} CH X_{2} CH X_{2} NO X_{2} + NaBr$

Use aqueous NaNO₂ (or KNO₂) in DMF at 0 °C → 25 °C.
The reaction proceeds via an SN2 pathway because the substrate is primary and the nitrite ion is a good nucleophile.

A chemical reaction occurs as follows : CH3CH2CH2Br + aq. KOH B + KBr (A) a) Identify the compound 'B' in the above reaction. [1] b) Which substitution reaction mechanism occurs in the above…

(A) Identification of ‘B’

(B) Substitution mechanism

(C) Product with alcoholic KOH

(D) Synthesis of nitropropane from ‘A’

Discussion

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