CBSE Class 11 Chemistry Chapter 12: Organic Chemistry - Some Basic Principles and Techniques NCERT Solutions

To determine the hybridization state of each carbon atom, we examine the number of sigma bonds and lone pairs around it. A carbon atom with 4 sigma bonds is sp³ hybridized, with 3 sigma bonds is sp² hybridized, and with 2 sigma bonds is sp hybridized.

1. For $CH\_2=C=O$ (Ketene):

   The first carbon atom ($\backslash overset\{1\}\{C\}H\_2$) is bonded to two hydrogen atoms and the second carbon atom via a double bond. It forms 3 sigma bonds and 0 lone pairs, hence it is $sp^2$ hybridized.

   The second carbon atom ($\backslash overset\{2\}\{C\}$) is double bonded to the first carbon and double bonded to oxygen. It forms 2 sigma bonds and 0 lone pairs, hence it is sp hybridized.

2. For $CH\_3-CH=CH\_2$ (Propene):

   The first carbon atom ($\backslash overset\{1\}\{C\}H\_3$) is bonded to three hydrogen atoms and the second carbon atom via a single bond. It forms 4 sigma bonds and 0 lone pairs, hence it is $sp^3$ hybridized.

   The second carbon atom ($\backslash overset\{2\}\{C\}H$) is bonded to the first carbon, one hydrogen atom, and the third carbon atom via a double bond. It forms 3 sigma bonds and 0 lone pairs, hence it is $sp^2$ hybridized.

   The third carbon atom ($\backslash overset\{3\}\{C\}H\_2$) is bonded to two hydrogen atoms and the second carbon atom via a double bond. It forms 3 sigma bonds and 0 lone pairs, hence it is $sp^2$ hybridized.

3. For $(CH\_3)\_2CO$ (Acetone):

   The two methyl carbon atoms ($CH\_3$) are each bonded to three hydrogen atoms and the central carbon atom via single bonds. Each of these forms 4 sigma bonds and 0 lone pairs, hence they are $sp^3$ hybridized.

   The central carbon atom ($C$) is double bonded to oxygen and single bonded to the two methyl groups. It forms 3 sigma bonds and 0 lone pairs, hence it is $sp^2$ hybridized.

4. For $CH\_2=CHCN$ (Acrylonitrile):

   The first carbon atom ($\backslash overset\{1\}\{C\}H\_2$) is double bonded to the second carbon and single bonded to two hydrogen atoms. It forms 3 sigma bonds and 0 lone pairs, hence it is $sp^2$ hybridized.

   The second carbon atom ($\backslash overset\{2\}\{C\}H$) is double bonded to the first carbon and single bonded to one hydrogen atom and the third carbon atom via a triple bond. It forms 2 sigma bonds and 0 lone pairs, hence it is sp hybridized.

   The third carbon atom ($\backslash overset\{3\}\{C\}\; \backslash equiv\; N$) is triple bonded to nitrogen and single bonded to the second carbon atom. It forms 2 sigma bonds and 0 lone pairs, hence it is sp hybridized.

5. For $C\_6H\_6$ (Benzene):

   In benzene, each of the six carbon atoms is bonded to two other carbon atoms and one hydrogen atom. Each carbon atom forms two single bonds and one double bond (in resonance structures), resulting in 3 sigma bonds and 0 lone pairs. Therefore, all 6 carbon atoms in benzene are $sp^2$ hybridized.

Sigma ($\backslash sigma$) bonds are formed by the head-on overlap of atomic orbitals, while pi ($\backslash pi$) bonds are formed by the sideways overlap of p-orbitals. Single bonds are always sigma bonds. Double bonds consist of one sigma and one pi bond. Triple bonds consist of one sigma and two pi bonds.

1. $C\_6H\_6$ (Benzene):

   Benzene has a cyclic structure with alternating double bonds (in resonance). It contains 6 C–C sigma ($\backslash sigma\_\{C-C\}$) bonds, 6 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds, and 3 pi ($\backslash pi\_\{C-C\}$) bonds due to the delocalized pi electron system.

2. $C\_6H\_\{12\}$ (Cyclohexane):

   Cyclohexane is a saturated cyclic hydrocarbon. It contains 6 C–C sigma ($\backslash sigma\_\{C-C\}$) bonds forming the ring and 12 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds. There are no pi bonds in cyclohexane.

3. $CH\_2Cl\_2$ (Dichloromethane):

   This molecule has a central carbon atom bonded to two hydrogen atoms and two chlorine atoms via single bonds. It contains 2 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds and 2 C–Cl sigma ($\backslash sigma\_\{C-Cl\}$) bonds. There are no pi bonds.

4. $CH\_2=C=CH\_2$ (Allene):

   Allene has a central carbon atom double-bonded to two other carbon atoms. The first and third carbon atoms are $sp^2$ hybridized, and the central carbon atom is sp hybridized. It contains 2 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds, 2 C=C sigma ($\backslash sigma\_\{C=C\}$) bonds, and 2 pi ($\backslash pi\_\{C=C\}$) bonds (one from each double bond).

5. $CH\_3NO\_2$ (Nitromethane):

   Nitromethane contains a methyl group and a nitro group. The C–N bond in the nitro group is a double bond (one sigma, one pi) due to resonance. It has 3 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds, 1 C–N sigma ($\backslash sigma\_\{C-N\}$) bond, 1 N–O sigma ($\backslash sigma\_\{N-O\}$) bond, and 1 N=O pi ($\backslash pi\_\{N=O\}$) bond (or delocalized pi electrons).

6. $HCONHCH\_3$ (N-Methylformamide):

   This molecule contains a formamide group. The C=O bond is a double bond. It has 1 C–H sigma ($\backslash sigma\_\{C-H\}$) bond, 1 N–H sigma ($\backslash sigma\_\{N-H\}$) bond, 1 C–N sigma ($\backslash sigma\_\{C-N\}$) bond, 1 C=O sigma ($\backslash sigma\_\{C=O\}$) bond, 1 C=O pi ($\backslash pi\_\{C=O\}$) bond, and 3 C–H sigma ($\backslash sigma\_\{C-H\}$) bonds in the methyl group.