Hydrocarbons (Alkane, Alkene and Alkyne) MCQ Questions & Answers in Organic Chemistry | Chemistry

$${\text{Stability of alkene}} \propto \frac{1}{{{\text{Heat of hydrogenation of alkene}}}}$$
Stability of an alkene depends upon the heat of hydrogenation of an alkene. Therefore, lower the heat of hydrogenation of an alkene, higher will be stability.
Order of stability is $$trans - but - 2 - ene{\text{ }} > {\text{ }}cis - but - 2 - ene{\text{ }} > {\text{ }}but - 1 - ene.$$
Heat of hydrogenation $$(kJ/mol)$$   are 115.5, 119.6 and 126.8 respectively.

In presence of $$Na$$  and liquid $$N{H_3}\,liq \cdot N{H_3}.\,\,Trans$$     addition of hydrogen on alkene occurs.
\[\underset{\begin{smallmatrix} \,\,\,\text{but 2-yne} \\ \text{(2 -butyne)} \end{smallmatrix}}{\mathop{C{{H}_{3}}-C\equiv C-C{{H}_{3}}}}\,\xrightarrow[\text{Birch reduction}]{Na,\,liq\cdot N{{H}_{3}}}\]

Nitration takes place in presence of concentrated $$HN{O_3} + \,$$  concentrated $${H_2}S{O_4}.$$
In strongly acidic nitration medium, the amine is converted into anilinium ion $$\left( { - N{H_2}^ + } \right);$$   substitution is thus controlled not by $${ - N{H_2}}$$  group but by $$ - N{H_3}^ + $$  group which, because of its positive charge, directs the entering group to the meta-position instead of ortho, and para.

$$n$$ - Butane, $$\mathop C\limits^1 {H_3}\,\,\mathop C\limits^2 {H_2}\,\,\mathop C\limits^2 {H_2}\,\,\mathop C\limits^1 {H_3}$$     has two different hydrogen atoms, marked by 1 and 2.

The heat of hydrogenation of an alkene depends upon its stability. Higher the stability, lower the heat of hydrogenation. Since $$C{H_2} = C{H_2}$$   has no substituent, it is the least stable alkene and hence has the highest heat of hydrogenation.

The order of reactivity of the hydrogen halides is $$HI > HBr > HCl.$$     As the size of halogen increases, the strength of $$H - X$$  bond decreases and hence, reactivity increases.

$$NaN{H_2}$$  is used to distinguish between 1-$$butyne$$  and 2-$$butyne.$$
\[C{{H}_{3}}C{{H}_{2}}C\equiv CH\xrightarrow{NaN{{H}_{2}}}\]      \[C{{H}_{3}}C{{H}_{2}}C\equiv CNa+\frac{1}{2}{{H}_{2}}\uparrow \]
\[C{{H}_{3}}-C\equiv C-H\xrightarrow{NaN{{H}_{2}}}\]      \[\text{no reaction}\]