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

Step 1: SN1 reactivity and halide type.
SN1 rate is controlled by carbocation stability. 2-Bromo-2-methylbutane forms a tertiary carbocation $(CH_3)_2C^+CH_2CH_3$ (highly stable), whereas 1-bromopentane forms an unstable primary carbocation. Therefore 2-bromo-2-methylbutane is more reactive toward SN1. In the given compound, Cl is bonded to a $sp^2$ carbon of a $C=C$ bond, making it a vinylic halide.
Step 2: Storage of chloroform.
In air and light, chloroform is oxidised to phosgene ($COCl_2$), a highly toxic gas: $2CHCl_3 + O_2 \rightarrow 2COCl_2 + 2HCl$. Dark coloured bottles prevent light from reaching the chloroform and inhibit this oxidation. Ethanol is also added as a stabiliser.
Step 3: Ambident nucleophiles.
Nucleophiles with two different atoms capable of acting as the attacking site are called ambident nucleophiles. Example: $CN^-$ can attack via carbon (giving nitrile $R$-$CN$) or via nitrogen (giving isocyanide $R$-$NC$); $NO_2^-$ can attack via N or O.
Step 4: Racemic mixture.
A racemic mixture is an equimolar mixture of the $(+)$-enantiomer and the $(-)$-enantiomer of a chiral compound. Both enantiomers rotate plane-polarised light by equal amounts in opposite directions, so the net optical rotation is zero and the mixture is optically inactive.