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Believe it or not, the drug paracetamol, the fibre nylon, and the proteins in your muscles have something in common: they are all examples of amides.This article is about amides in organic chemistry.We'll start by defining amides. We'll be taking a look at their functional group, general formula, and structure.We'll then find out about amide nomenclature.After that, we'll look at how you produce…
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Jetzt kostenlos anmeldenBelieve it or not, the drug paracetamol, the fibre nylon, and the proteins in your muscles have something in common: they are all examples of amides.
In organic chemistry, you might have previously come across Amines. These are organic molecules with the amine functional group, -NH2. Amides are molecules that are similar to amines. They contain the amine group, -NH2, bonded to the carbonyl group, C=O. This is known as the amide functional group.
Amides are organic molecules with the amide functional group, -CONH2. This consists of a carbonyl group bonded to an amine group.
Check out Amines and The Carbonyl Group for more information about these two functional groups.
We now know that amides contain a carbonyl group, C=O, bonded to an amine group, -NH2. This gives amides the general formula RCONH2. Here, R represents an organic group joined to the other side of the carbonyl group.
The general formula for an amide given above is actually the formula of a primary amide. You can also get secondary and tertiary amides, which are also known as N-substituted amides. In these cases, either one or both of the hydrogen atoms attached to the nitrogen atom are replaced by other organic R groups. This gives secondary and tertiary amides the general formulae RCONR'H and RCONR'R'', respectively. However, we'll be focusing mostly on primary amides.
Let's use our new knowledge of amides to draw their structure. Here is an example of an amide.
Note the carbonyl group on the left, with its C=O double bond, and the amine group on the right. Because this is a primary amide, the nitrogen atom is bonded to two hydrogen atoms and no other R groups.
We can expand on the structure of amides by showing their polarity. You might know that both the carbonyl and the amine group are polar. This makes amides polar as well. The carbon atom in the carbonyl group is always partially positively charged, whilst the oxygen atom is partially negatively charged. Meanwhile, the nitrogen atom in the amine group is partially negatively charged, whilst the hydrogen atoms are partially positively charged.
Moving on, let's look at amide nomenclature.
Naming primary amides is fairly simple. It all depends on the R group attached to the carbonyl group. In fact, it is very similar to naming carboxylic acids.
To name primary amides, we follow these steps.
Let's look at an example.
Name the following amide:
Applying the nomenclature rules to our example above, we can see that the longest carbon chain is three carbon atoms long. This gives it the root name -propan. If we number the carbon atoms starting from the carbon in the carbonyl group, we can see that there is a methyl group attached to carbon 2. This gives us the final name of 2-methylpropanamide.
You should remember from earlier in the article that secondary and tertiary amides have additional R groups attached to their nitrogen atom. To indicate these R groups, we use additional prefixes, indicated by the letter N-. Here's an example.
Name the following amide:
Once again, the longest carbon chain is three carbon atoms long. This gives the amide the root name -propan-. There is also a methyl group attached to the nitrogen atom. We show this using the prefix methyl-, preceded by the letter N-. This molecule's name is therefore N-methylpropanamide.
Next, let's move on to look at the production of amides. You need to know about two similar reactions:
The mechanism for these two reactions is covered in more depth in Acylation.
Reacting an acyl chloride with ammonia (NH3) produces a primary amide and ammonium chloride. This is a nucleophilic addition-elimination reaction. It is also a condensation reaction, as it releases a small molecule in the process. Here, that small molecule is hydrochloric acid (HCl). The hydrochloric acid then reacts with another molecule of ammonia to form ammonium chloride (NH4Cl).
For example, reacting ethanoyl chloride (CH3COCl) with ammonia (NH3) produces ethanamide (CH3CONH2)and hydrochloric acid, which further reacts with another molecule of ammonia to form ammonium chloride (NH4Cl).
Reacting an acyl chloride with a primary amine produces a secondary amide, also known as an N-substituted amide. Once again, this is an example of a nucleophilic addition-elimination reaction. It is also a condensation reaction, releasing hydrochloric acid in the process. The hydrochloric acid reacts with another molecule of the primary amine to form an ammonium salt.
For example, reacting ethanoyl chloride (CH3COCl) with methylamine (CH3NH2) produces N-methylethanamide (CH3CONHCH3) and methylammonium chloride (CH3NH3Cl):
Similarly, reacting an acyl chloride with a tertiary amine produces an amide with two N-substitutes.
You can also produce amides in the reaction between a carboxylic acid and either ammonia or an amine. You first react the carboxylic acid with solid ammonium carbonate to produce an ammonium salt. This turns into an amide when you heat it. However, this method has several disadvantages. It is much slower than the reaction between an acyl chloride and either ammonia or an amine, and it doesn't go to completion. This results in a lower yield.
Wondering how amides react? Let's explore that next. You need to know about two different reactions:
We'll also touch on amide basicity.
First up, let's look at what happens when you react an amide with an aqueous acid or alkali. You actually produce a carboxylic acid and either ammonia or an amine, depending on whether your amide is primary, secondary, or tertiary. This is a hydrolysis reaction and requires heating. The acid or alkali then reacts with the products formed.
Here are a couple of examples. Heating ethanamide (CH3CONH2) with aqueous hydrochloric acid (HCl) produces ethanoic acid (CH3COOH) and ammonia (NH3), which further reacts to form ammonium chloride (NH4Cl):
The hydrochloric acid acts as a catalyst in the first part of the reaction, as it isn't changed or used up in the reaction. However, it is involved in the second part of the reaction, when it turns ammonia into ammonium chloride.
Heating ethanamide with aqueous sodium hydroxide (NaOH) also produces ethanoic acid and ammonia. The ethanoic acid further reacts to form sodium ethanoate (CH3COONa):
Here, the amide reacts directly with the alkali. This means that, unlike in the reaction with acid that we saw above, the alkali is a reactant, not a catalyst.
You can use the reaction between an amide and an alkali to test for amides. Heating an amide with sodium hydroxide produces ammonia gas, which turns red litmus paper blue. It is also recognisable by its distinct pungent smell.
Next up, let's consider what happens when you reduce an amide using a strong reducing agent such as lithium tetrahydridoaluminate, LiAlH4. The reaction gets rid of the oxygen atom in the amide's carbonyl group and replaces it with two hydrogen atoms. This reaction takes place at room temperature in dry ether and also produces water.
For example, reducing methanamide (HCONH2) with LiAlH4 produces methylamine (CH3NH2) and water:
You might know that amines act as weak bases. This is because the nitrogen atom in their amine group is able to pick up a hydrogen ion from solution using its lone pair of electrons. However, despite also containing an amine group, amides aren't basic. This is because they contain a carbonyl group, C=O. The carbonyl group is extremely electronegative and draws electron density towards it, reducing the attractive strength of nitrogen's lone pair of electrons. Therefore, amides don't act as bases.
Knowing what amides are and how they react is all well and good, but how does that apply to real life? Here are some examples of amides and their uses.
You should now feel confident at defining amides and giving their general formula and structure. You should be able to describe how they are formed, as well as how they react. Finally, you should be able to name some common examples of amides.
Amides are formed in the nucleophilic addition-elimination reaction between an acyl chloride and either ammonia or a primary amine. This is also a condensation reaction.
Examples of amides include proteins, paracetamol, urea, and nylon.
Amides are used in the pharmaceutical industry. They also make up all proteins and enzymes. In addition, many synthetic fibres such as nylon and Kevlar are made from amides.
Amides can be primary, secondary, or tertiary. Primary amides have the general formula RCONH2, secondary amides have the general formula RCONHR’ and tertiary amides have the general formula RCONR’R’’. Secondary and tertiary amides are also known as N-substituted amides.
Amines are molecules with the amine functional group, -NH2. Amides also have the amine functional group, but in this case it is directly bonded to a carbonyl group, C=O. This creates the amide functional group: -CONH2.
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