Q1-13P - Organic Chemistry (Mcmurry)

Question

Draw a line-bond structure for propyne, ${\mathbf{CH}}_3\mathbf{C}\equiv \mathbf{CH}$ . Indicate the hybridization of the orbitals on each carbon, and predict a value for each bond angle.

Step-by-Step Solution

Answer

The line-bond structure of propyne is represented. In propyne, C-1 is ${\text{sp}}^{\text{3}}$ hybridized while C-2 and C-3 both are hybridized. Each bond angle at C-1 is ${\text{109}}^{\text{o}}\text{28'}$, each bond angle at C-2 and C-3 is $\text{180°}$.

Line bond structure- Propyne

Step 1: Line-Bond structure

Line-bond structure is a notation used to conveniently represent organic compounds. In this representation, the bonds between carbon and carbon (C-C) are shown by lines and all the atoms except carbon and hydrogen are shown. In the line-bond structure, the lines represent the bonds between two carbon atoms, vertices and ends represent the carbon atom itself. The number of hydrogen atoms at each carbon atom is found using the tetravalency of carbon.

For example: Propane, line bond structure of propane is represented as-

Line bond structure-Propane

Step 2: Line-bond structure of propyne

Line bond structure- Propyne

In propyne, C-1 is having 3 hydrogen atoms constituting ${\text{-CH}}_{\text{3}}$ group, shown at left end of the line-bond structure. 

C-2 is bonded to C-1 on one side ( shown by a line) and to C-3 by a triple bond on the other side (shown by 3 lines), so, a C atom is there at the vertex of the line-bond structure.

C-3, on one side is bonded to C-2 by a triple bond satisfying three valencies of C while the remaining one is satisfied by hydrogen atom thus there is CH at the other end (C-3)

Step 3: Hybridization

Mixing of  two or more atomic orbitals of comparable energies to generate two or more new hybrid orbitals of equivalent energies, is known as hybridization. Hybridization can be of different types ( ${\text{sp}}^{\text{3}}$,  ${\text{sp}}^2$, ${\text{sp}}^{}$ ) depending upon the atomic orbitals involved in mixing.

Four groups around the central atom will result in ${\text{sp}}^{\text{3}}$ hybridization, three groups around the central atom will result in ${\text{sp}}^{\text{2}}$ hybridization and two groups around the central atom will result in ${\text{sp}}^{}$ hybridization. For example: Propane,

  

                                          Propane                      

Similarly in propyne,          

                                            

                                    Propyne    

Step 4: Bond Angle

Bond angle is the angle formed by three adjacent bonded atoms.

In propane, since all the C atoms are ${\text{sp}}^{\text{3}}$ hybridized, the each bond angle in propane will be ${109}^{\circ }28\text{'}$.                                 

Similarly, C-1 in propyne is ${\text{sp}}^{\text{3}}$ hybridized, the bond angle will be ${109}^{\circ }28\text{'}$, while C-2 and C-3 are both  ${\text{sp}}^{}$ hybridized, each bond angle will be $180°$.