why would acyl halides give higher yields of amides compared to carboxylic acids or esters on reaction with NH3?
The first step that takes place in the amide formation reaction, is that the :NH3 molecule (the semicolon represents the lone pair) donates its lone pair to the carbon atom (which is connected to the oxygen atom), forming a bond.
The more electrophilic (wanting an electron/having a low electron density) the carboxyl/carbonyl carbon is, the easier this will be.
Now, in acyl halides, the halide ion connected to the carbon atom is electron withdrawing, and it hence increases the tendency of the carbon atom to accept a lone pair (makes it more electrophilic). Since halide atoms do this better than oxygen atoms in carboxylic acids or esters, the reaction is easier in acyl halides and gives a higher yield.
Well, it's all about the "halides" and the "ha-laughs"! When acyl halides react with NH3, they have a halogen (like chlorine or bromine) that acts as a leaving group. This makes it easier for NH3 to attack and form the amide.
But carboxylic acids and esters are not as "hal-larious"! They don't have a good leaving group, so it's harder for NH3 to come in and make the reaction happen. In other words, they're a little "ester-bating" the reaction's progress.
So, the acyl halides steal the show when it comes to making amides. They bring the halogens along for a chemical comedy act, while carboxylic acids and esters are left wondering, "Why doesn't anyone laugh at our chemistry jokes?"
Acyl halides generally provide higher yields of amides compared to carboxylic acids or esters when reacting with ammonia (NH3) due to two main reasons:
1. Reactivity: Acyl halides are more reactive than carboxylic acids or esters. This higher reactivity is attributed to the presence of a highly polarized carbon-oxygen bond in esters and carboxylic acids, while acyl halides contain a polarized carbon-halogen bond. The polarized carbon-halogen bond makes acyl halides more susceptible to nucleophilic attack by ammonia, resulting in faster reaction rates and higher yields of amides.
2. Leaving Group Ability: Acyl halides have better leaving groups compared to carboxylic acids or esters. When ammonia attacks the acyl halide, the leaving group (halide ion) is easily expelled due to its high electronegativity and relatively large size. On the other hand, in carboxylic acids and esters, the leaving group (hydroxide ion or alkoxide ion) is less electronegative and smaller, making it less favorable to leave. As a result, the reaction proceeds more smoothly with acyl halides, leading to higher yields of amides.
Overall, the combination of higher reactivity and better leaving group ability in acyl halides makes them more favorable for the synthesis of amides when reacting with ammonia.
Acyl halides (also known as acid halides) react with ammonia (NH3) to form amides, whereas carboxylic acids and esters do not readily undergo this reaction. This preference is due to the reactivity and nucleophilicity of the reaction components.
To understand why acyl halides give higher yields of amides compared to carboxylic acids or esters, we need to consider the mechanism of the reaction.
Acyl halides are highly reactive because the halide group (such as chloride or bromide) is a good leaving group. When an acyl halide reacts with ammonia, the nucleophilic nitrogen atom of ammonia attacks the carbonyl carbon of the acyl halide, resulting in the formation of a tetrahedral intermediate. The halide ion then leaves, and an amide is formed.
On the other hand, carboxylic acids and esters have lower reactivity due to the nature of their leaving groups. The hydroxyl group in carboxylic acids and the alkoxy group in esters are not as good leaving groups as halides. Therefore, the nucleophilic attack by ammonia on the carbonyl carbon is less favored for carboxylic acids and esters.
Additionally, carboxylic acids can undergo a competing reaction called decarboxylation, where carbon dioxide (CO2) is eliminated instead of forming an amide. This further decreases the yield of amides from carboxylic acids.
To summarize, acyl halides give higher yields of amides compared to carboxylic acids or esters on reaction with NH3 because acyl halides have more reactive leaving groups (halides) and are less prone to competing reactions like decarboxylation.