Question

Identify the molecules which have sp- hybridized carbon(s) from the list given under. I. Ethane nitrile \(\quad\) II. Buta-1, 3-diene \(\quad\) III. Propa-1, 2-diene, IV. Ethyne \(\quad\) V. Benzene

• A. I, II, III only
• B. II, III, IV only
• C. III, IV, V only
• D. I, III, IV only

Hint:

\textbf{sp$^3$ carbon:} Forms 4 single bonds (e.g., alkanes, CH$_3$- in nitrile).
\textbf{sp$^2$ carbon:} Forms 1 double bond and 2 single bonds (e.g., alkenes, benzene, terminal carbons of allene).
\textbf{sp carbon:} Forms 1 triple bond and 1 single bond (e.g., alkynes, nitrile carbon) OR forms 2 double bonds (e.g., central carbon of allenes, CO$_2$).
Draw the structure or recall the bonding pattern for each molecule to determine hybridization.

Answer 1

The Correct Option is D

An $sp$-hybridized carbon atom is typically involved in forming either: \begin{itemize} \item One triple bond and one single bond (e.g., in alkynes, nitriles). \item Two double bonds (e.g., in allenes like propa-1,2-diene, or in CO$_2$). \end{itemize} An $sp$-hybridized carbon atom has linear geometry (bond angles $180^\circ$). Let's analyze each molecule: I. Ethane nitrile (Acetonitrile): CH$_3$-C$\equiv$N \begin{itemize} \item The carbon atom of the methyl group (-CH$_3$) is $sp^3$ hybridized (forms 4 single bonds). \item The carbon atom of the nitrile group (-C$\equiv$N) is bonded to the CH$_3$ group by a single bond and to the N atom by a triple bond. This carbon is $sp$-hybridized. \end{itemize} So, Ethane nitrile has one $sp$-hybridized carbon. II. Buta-1, 3-diene: CH$_2$=CH-CH=CH$_2$ This molecule has two double bonds separated by a single bond (conjugated diene). Each carbon atom involved in a C=C double bond is $sp^2$-hybridized. All four carbon atoms in buta-1,3-diene are $sp^2$-hybridized. So, Buta-1,3-diene does not have any $sp$-hybridized carbons. III. Propa-1, 2-diene (Allene): CH$_2$=C=CH$_2$ This is an allene, which has cumulated double bonds. \begin{itemize} \item The two terminal carbon atoms (C1 and C3, part of CH$_2$=) are $sp^2$-hybridized (each forms two single bonds to H and one double bond to the central C). \item The central carbon atom (C2, part of =C=) forms two double bonds, one with C1 and one with C3. This central carbon is $sp$-hybridized. \end{itemize} So, Propa-1,2-diene has one $sp$-hybridized carbon. IV. Ethyne (Acetylene): HC$\equiv$CH Each carbon atom forms one single bond (to H) and one triple bond (to the other C). Both carbon atoms in ethyne are $sp$-hybridized. So, Ethyne has two $sp$-hybridized carbons. V. Benzene: C$_6$H$_6$ Benzene is a cyclic molecule with a system of delocalized $\pi$ electrons. Each carbon atom in the ring forms two single bonds (to adjacent carbons) and one double bond (within the resonance structure depiction, effectively 1.5 bonds) and one single bond to a hydrogen atom. All six carbon atoms in benzene are $sp^2$-hybridized. So, Benzene does not have any $sp$-hybridized carbons. Molecules with $sp$-hybridized carbon(s) are: I (Ethane nitrile) III (Propa-1,2-diene) IV (Ethyne) This corresponds to option (d). \[ \boxed{\text{I, III, IV only}} \]