Question

Which of the following sets is in the correct order regarding the property mentioned against them?

	Sets	Property
I)	$\ce{NCCH2 COOH > F CH2 COOH > H3CCH_2 COOH}$	acidity
II)	$\ce{CH3CH2CHO > PhCOCH3 > PhCHO }$	reactivity
III)	$\ce{H3COCH2CH3 < H3CCH2CHO < H3CCH_2CH2OH}$	boiling points

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

Answer 1

Answer: (B)

I. Key Idea $(-I)$ group, if are present along with $- COOH$ group in an acid, they increases the acidic property while $(+) I$ groups or groups show hyperconjugation will decreases the acidic property.

(a) $NCCH _{2} COOH$ : Contain, $NC$ (i.e. $\left.-I\right)$ group along with $- COOH$, thus will increases the acidic nature.

(b) $FCH _{2}- COOH$ : Contain, $F$ (i.e. $\left.-I\right)$ group thus increases the acidic nature. Between $NCCH _{2} COOH$ and $FCH _{2} COOH , NCCH _{2} COOH$

is more acidic due to stronger $(-) I$ effect.

(c) $H _{3} CCH _{2} COOH :$ Contain, $CH _{3}$ group which show hyperconjugation effect and this is least acidic. Hence, the given order is correct.

(II) Key Idea (i) Structures which show more resonance structures or hyperconjugation among aldehydes are less reactive.

(ii) Aldehydes are more reactive than ketones.

(a) $CH _{3} \cdot CH _{2} \cdot CHO$ is an aldehyde in which $CH _{3}- CH _{2}-$ group show hyperconjugation effect.

(b) $PhCOCH _{3}$, i.e. $^{Ph}_{CH_3} > CO$ is a ketone and is less reactive than $CH _{3} CH _{2} \cdot CHO$ and $PhCHO$ (as these are aldehydes).

(c) $PhCHO$, i.e. $^{Ph}_{H} > CO$ is an aldehyde and is less reactive than $CH _{3} CH _{2}- CHO$ due to resonance effect of $Ph$-group. Thus, correct order is

$CH _{3} CH _{2} \cdot CHO > PhCHO > PhCOCH _{3}$ and the given

order is not correct.

(III) Key Idea Boiling point of alcohols (due to ability to form $H$ -bond $)$ are higher than that of aldehydes which are more than that of ketones. (due to less steric-hinderance and more surface area in aldehyde group, having same number of C-atoms)



(b) $H_3C \cdot CH_2 \cdot CHO$ is an aldehyde, i.e.



(c) $H_3C - CH_2 - CH_2 - OH$ is an alcohol thus the given order for boiling point is correct