Question

Classify the following reactions in one of the reaction type studied in this unit. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}+\mathrm{Br}^{-}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC}-\mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}^{-}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Short answer

(a) Substitution, (b) Addition, (c) Elimination, (d) Substitution.

Step-by-step solution

Analyze Reaction (a)

The reaction is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} + \mathrm{Br}^{-} \). This involves the substitution of the bromide ion, \( \mathrm{Br}^{-} \), by the thiolate ion, \( \mathrm{HS}^{-} \). Therefore, it is a nucleophilic substitution reaction.

Analyze Reaction (b)

The reaction is \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C} = \mathrm{CH}_{2} + \mathrm{HCl} \rightarrow \left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC} - \mathrm{CH}_{3} \). A molecule of HCl is added across the double bond, transforming it into a single bond. This type of reaction is known as an addition reaction.

Analyze Reaction (c)

The reaction is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2} = \mathrm{CH}_{2} + \mathrm{H}_{2} \mathrm{O} + \mathrm{Br}^{-} \). Here, the ethyl bromide is converted into ethylene by the elimination of HBr and water. This is an elimination reaction.

Analyze Reaction (d)

The reaction is \( \left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH} + \mathrm{HBr} \rightarrow \left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} + \mathrm{H}_{2} \mathrm{O} \). This reaction involves substitution of an alcohol group by a bromine atom, indicating a nucleophilic substitution reaction.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a vital concept in organic chemistry involving the replacement of an atom or group in a molecule by a nucleophile. A nucleophile is an electron-rich species that seeks positively charged or electron-deficient sites.
In a typical nucleophilic substitution reaction, the nucleophile donates a pair of electrons to form a new covalent bond with the carbon atom, displacing another group known as the leaving group.
For example, in reaction (a): \[ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} + \mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH} + \mathrm{Br}^{-} \]The bromide ion (\(\mathrm{Br}^{-}\)) is displaced by the thiolate ion (\(\mathrm{HS}^{-}\)), which acts as the nucleophile.

• Types: There are two primary mechanisms for nucleophilic substitution: \(S_N1\) and \(S_N2\).
• \(S_N2\) Mechanism: Occurs in a single step with simultaneous bond-making and bond-breaking. Common in primary alkyl halides.
• \(S_N1\) Mechanism: Involves a two-step process with the formation of a carbocation intermediate. Common in secondary and tertiary alkyl halides.

Addition Reaction

An addition reaction is characterized by the addition of atoms or groups to a molecule, typically across a double or triple bond, converting it into a single bond. This reaction often increases the molecular complexity.
In the context of the given exercise, consider reaction (b):\[\left(\mathrm{CH}_3\right)_2 \mathrm{C} = \mathrm{CH}_2 + \mathrm{HCl} \rightarrow \left(\mathrm{CH}_3\right)_2 \mathrm{ClC} - \mathrm{CH}_3\]Here, a molecule of hydrogen chloride (HCl) adds across the double bond of the alkene, turning it into a single bond alkane.

• Types: Addition reactions can be electrophilic, nucleophilic, or free-radical.
• Electrophilic Addition: Common in alkenes and alkynes where an electrophile adds to the unsaturated carbon.
• Regioselectivity: The orientation of these additions is often governed by Markovnikov’s rule, where the hydrogen atom is added to the carbon with more hydrogens.

Elimination Reaction

Elimination reactions are processes where a molecule loses atoms or groups of atoms, resulting in the formation of a double or triple bond. These reactions are crucial for constructing unsaturated compounds like alkenes and alkynes.
Let's examine reaction (c): \[\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br} + \mathrm{HO}^- \rightarrow \mathrm{CH}_2 = \mathrm{CH}_2 + \mathrm{H}_2 \mathrm{O} + \mathrm{Br}^-\]In this case, the reaction removes one bromine atom and one hydrogen atom from the molecule, creating an alkene. The result is the formation of a double bond, a signature of elimination reactions.

• Types: The common mechanisms include \(E1\) and \(E2\) mechanisms.
• \(E2\) Mechanism: A single-step process with the concurrent removal of hydrogen (proton) and leaving group forming a double bond.
• \(E1\) Mechanism: Involves a two-step process with the formation of a carbocation intermediate.
• Dehydration and Dehydrohalogenation: Typical examples include removal of water (dehydration) or hydrogen halides (dehydrohalogenation) from alcohols and alkyl halides respectively.