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Aldehydes and Ketones


This is MCAT Organic Chemistry 2. We'll be covering aldehydes and ketones. Now an aldehyde has this carbonyl directly bonded to a hydrogen. When we're naming aldehydes we replace the e for an alkane with al or formyl for the prefix. For example, methane becomes methanal.

This is written as a single carbon attached to a hydrogen, another hydrogen, and the double bond to oxygen. Now the common name for this is formaldehyde or formaldehyde. For ethane, it becomes ethanal. And this then is our methyl group attached to our carbon. This one is commonly known as acetaldehyde.

And we mentioned that we can use formyl for the prefix. So if we have 2-methybutanal, this is also the same thing as 2-formylbutane. Let's draw this up so we can se this. We have our first carbon, our second, our third, our fourth, or we can call this one our first one. If we number this one as the first, 1, 2, 3, 4, on our second position, we have a methyl group, 2-methylbutanal.

Because the carbon chain is a butane group. If we number it here, 1, 2, 3, 4, we have a butane that has a formyl group on the 2 position. So it'd be a 2-formylbutane. So there are two ways to name this compound. And aldehyde properties include an increased polarity over the alkane.

Because of this carbonyl, we have a slightly negative oxygen and a slightly positive carbon. This polarity gives the aldehyde a slightly higher boiling point than the alkane for those with the same molecular weight. Now this polarity will also give us a slightly higher solubility in water. And another important thing to remember is that because the carbonyl here is sp2 hybridized, we have bond angles of 120 degrees.

Now remember that things like polarity, boiling point, solubility differences, these are all diminished as the chain length increases. Because as this chain length increases, molecule acts less and less like an aldehyde and more like an alkane. So for longer chain molecules, the effect of a particular functional group drops dramatically.

Now a ketone has this same carbonyl functionality. But instead of having a hydrogen attached to this carbon, there are two R or alkyl groups on either side of it. Now when we name a ketone, we replace the e with one or oxo if we're using it as a prefix. So, for example, propane would become propanone.

Now notice that C3 is the first time we can get a ketone. With ethane, If we try to turn that into a ketone, we got acetaldehyde. And with methane, once we turn that into a ketone, instead, we get formaldehyde. So propane is the first alkane that can actually have a ketone group. What are some common names of propanone here? Well, we could call this 2-propanone, but the 2 is redundant.

Other names for it include acetone. And we name it by looking at the substituents attached to the carbonyl and call this dimethyl ketone. So another way to name ketones is to look at what's attached to the ketone group and to use those instead of using the backbone. So if we had something like this, we could say there's 1, 2, or an ethyl group.

1, 2, 3 or a propyl group. So this could be ethyl propyl ketone. Or 1, 2, 3, 4, 5, 6 since there are 6 carbons. And we have this ketone group on the 1, 2, 3 position, this could be 3-hexanone. But continuing on, butane will become a butanone.

Remember there are no isomers possible for a butanone. Because if we ship this to this carbon, it's the same. And if we put it along the ends, we'd get the aldehyde butanal. And last, 4-methylpentan-2-one or are using the one as a suffix could be rewritten as 4-methyl-2-oxopentane. If we draw this, methyl group carbonyl, we have 1, 2, 3, 4, 5.

If we use this as the ketone group, this is on the 2, our methyl group is on the 4. So it says 4-methylpentan-2-one. Or if we wanna label this as an oxo group, it becomes 4-methyl-2-oxopentane. Now ketone properties are similar to aldehyde properties. They have higher polarity than alkanes because of the carbonyl.

Which has slightly negative oxygen and slightly positive carbons. This leads into boiling points that are slightly higher than the alkanes. Their solubility is also higher. And again, they have a bond angle here of 120 degrees. And the last thing to point out is that both aldehydes and ketones have this carbonyl carbon, which is slightly positive.

This makes it a good nucleus for nucleophiles. And consequently, it is often the point of attack.

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