Question

(I) Which of the following is more reactive towards S$_N$1 reaction: 2-Bromo-2-methylbutane or 1-Bromopentane (II) What type of halide is present in the following compound: \[ \begin{array}{c} CH_3-CH-C=CH_2\\ |\\ CH_3 \end{array} \] \[ | \] \[ Cl \] (III) Why is chloroform stored in dark coloured bottles? (ii) Define the following terms: (I) Ambident Nucleophiles (II) Racemic mixture

Hint:

For S$_N$1 reactions: \[ \boxed{3^\circ \gt 2^\circ \gt 1^\circ} \] carbocation stability decides reactivity. Remember: \[ \boxed{\text{Halogen attached to }sp^2\text{ carbon}=\text{Vinylic halide}} \] Chloroform: \[ \boxed{\text{Light + oxygen }\rightarrow\text{ poisonous phosgene}} \]

Answer 1

Part (i):

(I) Reactivity towards S$_N$1 reaction The rate determining step of an S$_N$1 reaction is the formation of carbocation: \[ R-X\rightarrow R^+ + X^- \] The stability of carbocation determines the rate of reaction. The order of carbocation stability is: \[ 3^\circ \gt 2^\circ \gt 1^\circ \] For 2-bromo-2-methylbutane: \[ CH_3-C(Br)(CH_3)-CH_2-CH_3 \] Removal of bromide ion gives: \[ (CH_3)_2C^+CH_2CH_3 \] which is a tertiary carbocation. For 1-bromopentane: \[ CH_3CH_2CH_2CH_2CH_2Br \] removal of bromide ion gives a primary carbocation, which is highly unstable. Therefore: \[ \boxed{\text{2-Bromo-2-methylbutane is more reactive towards S}_N1} \]

(II) Type of halide In the given compound, chlorine is directly attached to a carbon of the double bond. The structure is: \[ CH_3-C(Cl)=CH_2 \] The carbon attached to chlorine is $sp^2$ hybridised. Therefore, it is classified as: \[ \boxed{\text{Vinylic halide}} \]

(III) Storage of chloroform Chloroform undergoes oxidation in presence of oxygen and sunlight. The reaction is: \[ 2CHCl_3+O_2 \rightarrow 2COCl_2+2HCl \] The product formed is phosgene gas: \[ COCl_2 \] which is highly poisonous. Therefore, chloroform is stored in dark coloured bottles to prevent the reaction with sunlight. A small amount of ethanol is also added as a stabiliser because it reacts with phosgene.

Part (ii): Definitions

(I) Ambident Nucleophiles Some nucleophiles have two different nucleophilic centres and can attack through either atom. Such nucleophiles are called ambident nucleophiles. Examples: \[ CN^- \] can attack through carbon or nitrogen. \[ NO_2^- \] can attack through nitrogen or oxygen.

(II) Racemic Mixture A racemic mixture contains equal amounts of two optical isomers: \[ (+)\text{-enantiomer} \] and \[ (-)\text{-enantiomer} \] The optical rotations of both isomers cancel each other, making the mixture optically inactive.

Final Answer:

(i) \[ \boxed{\text{2-Bromo-2-methylbutane is more reactive towards S}_N1} \] because it forms a stable tertiary carbocation. \[ \boxed{\text{The given compound is a vinylic halide}} \] \[ \boxed{\text{Chloroform is stored in dark bottles to prevent formation of poisonous phosgene gas}} \]

(ii) \[ \boxed{\text{Ambident nucleophiles: Nucleophiles having two attacking centres}} \] \[ \boxed{\text{Racemic mixture: Equal mixture of two enantiomers causing optical inactivity}} \]