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Organic Chemistry - Aldehydes and Ketones

Let us now talk a little about aldehydes and ketones. First of all, the C double bond O group is called a carbonyl group. Second of all, the carbonyl group is polar since oxygen is more electronegative than carbon. Therefore, aldehydes and ketones are slightly polar molecules 

In respect to solubility, aldehydes and ketones are like alcohols. Small sized aldehydes and ketones are soluble in water and the bigger ones are not. 

When it comes to melting points and boiling points remember that the greater the molecular weight, the higher the melting and boiling points; the more branching, the lower the boiling point. You should remember that since aldehydes and ketones are polar, they tend to have higher boiling points than alkanes and alkenes of similar molecular weight. On the other hand, they do not undergo hydrogen-bonding, which means that their boiling points are a little lower than alcohols of similar molecular weight. 

Nucleophile is an atom or a radical group that is anxious to get rid off electrons. It wants to give away some negative charge. So, nucleophile is basically the opposite of electronegativity, the opposite of electron affinity. When you see some atom or radical with an unshared pair of electrons think nucleophile. 

Aldehydes and ketones can gain a nucleophile because the carbonyl carbon is slightly positive, nucleophiles like it. And, aldehydes and ketones are prime targets for nucleophiles. A reaction in which a nucleophile attacks a carbonyl carbon first is called nucleophilic addition. So, when an aldehyde or a ketone undergoes nucleophilic addition here is what happens. The nucleophile itself becomes bonded to the carbonyl carbon. A hydrogen gets attached to the carbonyl oxygen and the double bond between C and O turns into a single bond. You need to know other nucleophilic addition reactions at the carboxyl bond. When an alcohol is added to an aldehyde, a hemi-acetyl is formed. If another alcohol is added, an acetyl is formed. If the alcohol is added to a ketone instead, a hemi-ketyl and ultimately, a ketyl is formed. 

Another example of a nucleophilic addition is an aldyl condensation in which two carbonyls condense to form an aldyl. Aldehydes can lose a hydrogen ion and get an OH group instead. If we take an aldehyde and expose it to oxygen in the presence of an oxidizing agent, the aldehyde will lose the hydrogen that is attached to its carbonyl carbon and gain an OH instead. That means the aldehyde has turned into a carboxylic acid. When this happens, we say that the aldehyde has been oxidized. Remember that the two oxidizing agents you will probably see are KMnO4 and K2CR2O7. Also, remember that ketones are less susceptible to nucleophilic addition and oxidation than are aldehydes. 

The carbons that are next to the carbonyl carbons are called, alpha carbons. If you take a hydrogen ion off an alpha carbon, you are left with a negatively-charged anion called a carbanion. Carbanion is a carbon chain compound that is missing a hydrogen ion. Now, when a carbanion is formed by removing a hydrogen ion from an alpha carbon, we end up with a resonant structure which is very stable. This is important because the resonant structure makes the alpha carbon more willing to give up a hydrogen atom. Now, the alpha hydrogen on an aldehyde is more acidic than the alpha hydrogen on a ketone. In other words, aldehydes are better at giving away hydrogens than are ketones. 

Carbonyls exist as tautomers. Tautomers are different forms of the same thing. The two tautomers for a carbonyl are the keto-form and enol-form. At equilibrium, both of these forms exist. So, this equilibrium is referred to as keto-enol tautomerism.