Greater is the stability of the substituted ammonium cation, stronger should be the corre sponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary ¿ secondary ¿ tertiary, which is opposite to the inductive effect-based order. Secondly, when the alkyl group is small, like the-CH3 group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than the-CH3 group, there will be steric hindrance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from-CH3 to-C2H5 results in a change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect, and steric hindrance of the alkyl group which decides the basic strength of alkyl amines in the aqueous medium.
Question: 1
Which among the following amines is most basic in aqueous medium?
In aqueous medium, the basicity of amines is influenced by several factors, including steric hindrance and the ability to stabilize the positive charge on the ammonium ion formed upon protonation. Primary amine (C2H5NH2) : Has two hydrogen atoms available for bonding, allowing for effective solvation and stabilization when protonated.- Secondary amine ((C2H5)2NH) : Has some steric hindrance but is still relatively basic.- Tertiary amine ((C2H5)3N) : Although it can stabilize a positive charge well, steric hindrance prevents effective interaction with protons in aqueous solution.- Ammonia (NH3) : Less basic than primary amines. Thus, in the aqueous medium, the most basic amine is the primary amine, C2H5NH2 .
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Question: 2
Which base among the following is least basic in aqueous medium?
To determine the least basic amine among the options provided, we analyze the effects of alkyl groups on basicity in aqueous medium: 1. Methylamine (CH3NH2) : A primary amine that is relatively basic due to less steric hindrance and effective solvation. 2. Dimethylamine ((CH3)2NH) : A secondary amine that is basic, but steric hindrance starts to affect its basicity. 3. Trimethylamine ((CH3)3N) : A tertiary amine where steric hindrance is significant, leading to reduced basicity compared to primary and secondary amines. 4. Ammonia (NH3) : Though not an amine with alkyl groups, it serves as a reference for basicity. In conclusion, trimethylamine ((CH3)3N) is the least basic in aqueous medium due to significant steric hindrance, which limits effective solvation and protonation.
In the gaseous phase, trimethylamine ((CH3)3N) is the most basic amine due to the strong electron-donating effect of the three methyl groups despite its steric hindrance.
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Question: 4
Which factor is not responsible for the basic strength of amines?
The basic strength of amines in aqueous medium is influenced by factors such as:
Inductive Effect: The electron-donating or withdrawing properties of alkyl groups impact the availability of the nitrogen's lone pair for protonation.
Solvation Effect: Better solvation with water molecules enhances basicity by stabilizing the protonated amine.
Steric Hindrance: Larger alkyl groups hinder proton access to the nitrogen, reducing basicity.
Conjugation Factor: This does not significantly affect the basic strength of amines, as conjugation involves pi bonds and does not influence protonation directly.
Thus, conjugation is not responsible for the basic strength of amines.
Hinsberg's reagent, benzenesulphonyl chloride, is used to differentiate primary, secondary, and tertiary amines based on their reaction products. Primary amines form soluble sulfonamides, secondary amines form sulfonamides that may be soluble or insoluble, and tertiary amines do not react significantly. Thus, benzenesulphonyl chloride is the correct reagent for this identification.
