Posted by: Mark Foreman | January 26, 2016

Spot the functional group

Dear Reader,

Some of you might be aware of a game played in UK newspapers called spot the ball, it is a photo of a sporting event where the ball has been painted out. The reader is invited to use their “skill and judgement” to work out where the ball is and then mark an X where it is. We will be doing something else which is “spot the functional group”. I am going to publish here a series of pictures of molecules. You need to spot the functional groups, name them and then consider what the answers are.

Before we get going it is important to understand that much of organic chemistry is about reactions of functional groups, but the functional groups can interact with each other. For example a hydroxyl group attached to a benzene ring is modified by the benzene ring. The pKa of the hydroxyl group goes down a lot, while the electron density in the pi cloud of the benzene ring increases a lot.

Now lets get started. Our first three molecules.

molecules1

Lets look at them and search for the functional groups, before we move onto the next ones.

molecules1a

Now we will go through the different functional groups which we have spotted.

The alcohol is a sp3 oxygen atom which bears an aliphatic carbon and a hydrogen atom. The chemistry of the alcohol is similar to that of the phenols but the acidity of the alcohols is much lower. Normally alcohols groups are attached to saturated (sp3) carbons, but if an alcohol group was to be attached to a sp2 carbon in an alkene (such as vinyl alcohol) then it is normal for it to be converted by keto-enol tautomerism to an aldehyde or in some cases a ketone.

The phenol is like the alcohol but the carbon is part of a benzene ring, the pi system of the benzene ring withdraws electron density from the oxygen thus increasing the acidity of the OH group. In some ways the phenols can be regarded as a special case where the keto-enol tautomerism has been reversed. If we were to consider the keto form of phenol (cyclohexa-2,4-dien-1-one) then we can see how by converting the ketone into an “alkene” and a hydroxyl group the molecule can have an aromatic ring. We will get onto aromatic rings later. When I write “benzene ring”, it is important to understand that other aromatic systems such as naphthalene exist. A hydroxy naphthalene such as naphthol should be regarded as a phenol. Even while it has a different aromatic system.

The ether is a sp3 oxygen which is attached to two carbon groups, in common with the hydroxyl group it can donate electron density to a benzene ring. It is possible for both carbon groups to be aromatic, both aliphatic or even one of each. If the ether oxygen is attached to an aliphatic unsaturated group such as a vinyl then it is stable. The “keto-enol tautomerism” reaction will not convert it to something else.

The alkene is reactive towards electrophiles as it has a pi cloud in the form of the pi orbitals of the double bond. It is important to understand that a double bond is a sigma bond and a pi bond. We will attack this group later, at least in an intellectual manner rather as a hoard of electrophiles. I wounder what my students would be like if some miricle was to convert them into atoms / molecules. I think that they are better off as humans.

The benzene ring is not a simple collection of three alkenes, it has a special stability. Through resonance the bonds between the carbons in the ring have a bond order of 1.5 rather than 1 or 2. We need to pay special attention to the benzene ring as it is so common but it also can do many interesting things.

The last group we have is the amino group, this can be regarded as an ammonia which has had one or more hydrogen replaced with a carbon based group such as a aryl (aromatic) or alkyl (saturated aliphatic) group.

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