What is the difference in chemical properties of propanal and propanone?
Answers
Answer:
(i) Propanone gives iodoform test while propanol does not give this test.
Propanone
CH
3
COCH
+3I
2
+2Na
2
CO
3
→
Yellowppt
CHI
3
+CH
3
COONa+2NaI+2CO
2
+H
2
O
(ii) Benzaldehyde (C
6
H
5
CHO) and acetophenone (C
6
H
5
COCH
3
) can be distinguished by iodoform test.
Acetophenone, being a methyl ketone on treatment with I
2
/NaOH undergoes iodoform reaction to give a yellow ppt. of iodoform. on the other hand, benzaldehyde does not give this test.
C
6
H
5
COCH
3
+3NaOI
Acetophenone
↓
C
6
H
5
COONa+
Iodoform
CHI
3
↓
+2NaOH
C
6
H
5
CHO
NaOI
No yellow ppt of iodoform
Benzaldehyde
(b)
(i) Methyl ter-butyl ketone < Acetone < Acetaldehyde
When HCN reacts with a compound, the attacking species is a nucleophile, CN
−
.
Therefore, as the negative charge on the compound increases, its reactivity with HCN decreases. In the given compounds, the +I effect increases as shown below. It can be observed the steric hindrance also increase in the same.
(ii) 4-Methoxybenzoic acid < Benzoic acid < 3, 4-Dinitrobenzoic acid
Electron-donating group decrease the strengths of acids, while electron-withdrawing groups increases the strengths of acids. As methoxy group is an electron-donating group, 4-ethoxybenzoic acid is a weaker acid than benzoic acid. Nitro group is an electron-withdrawing group and will increase the strengths of acids.
(iii) (CH
3
)
2
CHCOOH<CH
3
CH(Br)CH
2
COOH<CH
3
CH
2
CH(Br)COOH
After losing a proton, carboxylic acids gain a negative charge as shown:
R−COOH→R−COO
−
+H
+
Now, any group that will help stabilise the negative charge will increase the stability of the carboxyl ion and as a result, will increase the strength of the acid. Thus, groups having +I effect will decrease the strength of the acids and groups having -I effect will increase the strength of the acids. In the given compounds, −CH
3
group has +I effect and Br
−
group has −I effect. Thus, acid containing Br
−
are stronger.
The −I effect grows weaker as distance increase. Hence, CH
3
CH(Br)CH
2
COOH is weaker acid than CH
3
CH
2
CH(Br)COOH.
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