Question

Which of the following is incorrect statement ?

• A. bond angle in $CH_4$ is $109.5^o$
• B. $C-C$ and $C-H$ bond lengths are 154 pm and 112 pm respectively
• C. Isobutane < n - pentane < isopentane < neopentane (order of B.P)
• D. $C_6H_{14}$ has five structural isomers

Answer 1

Answer: (C)

Isobutane < n - pentane < isopentane < neopentane (order of B.P)

Answer 2

The question asks to identify the incorrect statement. Let's analyze each option:

1. Bond angle in $CH_4$ is $109.5^o$: Methane ($CH_4$) has a tetrahedral geometry with an ideal bond angle of $109.5^\circ$. This statement is correct.
2. $C-C$ and $C-H$ bond lengths are 154 pm and 112 pm respectively: Typical $C-C$ single bond length is around 153-154 pm, and $C-H$ single bond length is around 109-112 pm. These values are standard. This statement is correct.
3. Isobutane < n - pentane < isopentane < neopentane (order of B.P): The boiling points are: Isobutane (-11.7 °C), n-pentane (36.1 °C), isopentane (27.8 °C), neopentane (9.5 °C). The correct order is Isobutane < Neopentane < Isopentane < n-pentane. This statement is incorrect.
4. $C_6H_{14}$ has five structural isomers: The structural isomers of hexane ($C_6H_{14}$) are n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. There are indeed five structural isomers. This statement is correct. Therefore, the incorrect statement is the one regarding the order of boiling points.