A hydrocarbon A (molecular formula C5 H10 ) yields 2-methylbutane on catalytic hydrogenation. A

step 1 A hydrocarbon A, C5H10 yields to methylbutane on catalytic hydrogenation. A adds HBR in the pres

A hydrocarbon A (molecular formula C5 H10 ) yields...

A hydrocarbon A (molecular formula $\mathrm{C}_{5} \mathrm{H}_{10}$ ) yields 2-methylbutane on catalytic hydrogenation. A adds $\mathrm{HBr}$ (in accordance with Markownikoff's rule) to form a compound B which on reaction with silver hydroxide forms an alcohol $\mathrm{C}, \mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O} .$ Alcohol $\mathrm{C}$ on oxidation gives a ketone D. Deduce the structures of $A, B, C$ and $D$ and show the reactions involved.

Key Concepts

Hydrocarbon Structure Determination

Often in organic synthesis problems one is given only the molecular formula and the product after catalytic hydrogenation. In such cases the unsaturation is identified as a C=C double bond, and the fact that hydrogenation yields a specific branched alkane is used to deduce which alkene isomer (or precursor) must have been present. This requires matching the connectivity of the alkene with the saturated structural isomer.

Markovnikov Addition

Markovnikov’s rule governs how a protic acid (HX) adds to an unsymmetrical alkene. The rule states that the hydrogen atom attaches to the carbon that has more hydrogen substituents (or is less substituted), thereby generating the more stable carbocation intermediate. This regioselectivity determines the position at which the halogen ends up in the addition product.

Carbocation Rearrangement

When carbocations are generated during electrophilic addition, they may rearrange (typically via hydride or alkyl shifts) if a more stable carbocation can be formed. This rearrangement can lead to a product in which the atom added (such as bromine in an HX addition) is located at a position different from that suggested by a direct addition, and it is essential for correctly predicting subsequent reaction outcomes.

Nucleophilic Substitution Reactions

Alkyl halides can undergo nucleophilic substitution reactions with reagents such as silver hydroxide to form alcohols. The structure (primary, secondary or tertiary) of the alkyl halide influences whether the reaction proceeds via an SN2 or an SN1 mechanism. Recognizing the potential for such substitution reactions is important when planning multi?step syntheses.

Oxidation of Secondary Alcohols

Secondary alcohols are commonly oxidized to ketones under appropriate conditions. The ability to interconvert a secondary alcohol to a carbonyl group by oxidation is a key transformation in organic synthesis, and it aids in deducing product structures in reaction sequences given that only secondary alcohols (and not tertiary ones) are readily oxidized to ketones without cleavage of the carbon framework.

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