Cyclic ketones can achieve the perfect 120° angle at the carbonyl group only if the ring is at least six-membered. The smaller rings are ‘strained’ because the orbitals have to overlap at a less than ideal angle. For a four-membered ring, the actual angle is 90°, so there is 120° – 90° = 30° of strain at the carbonyl group. The effects of this strain on five-, four-, and three-membered rings are shown here.

But why should strain raise the frequency of a carbonyl group? It is evidently shortening and strengthening the C=O bond as it moves it towards the triple bond region (higher frequency), not towards the single-bond region (lower frequency). In a six-membered ring, the sp2 orbitals forming the σ framework around the carbonyl group can overlap perfectly with the sp3 orbitals on neighbouring carbon atoms because the orbital angle and the bond angle are the same. In a four-membered ring the orbitals do not point towards those on the neighbouring carbon atoms, but point too far out, effectively forcing the bonds to be bent and lowering the degree of overlap. Ideally, we should like the orbitals to have an angle of 90° as this would make the orbital angle the same as the bond angle. In theory it would be possible to have a bond angle of 90° if we used pure p orbitals instead of sp2 hybrid orbitals. The diagram in the margin shows this hypothetical situation. If we did this, we should leave a pure s orbital for the σ bond to oxy gen. This extreme is not possible, but a compromise is. Some more p character goes into the ring bonds—maybe they become s^0.8p^3.2—so that they can approach the 90° angle needed, and the same amount of extra s character goes into the σ bond to oxygen. The more s char acter there is in the orbital, the shorter it gets as s orbitals are much smaller than p orbitals.

مواضيع ذات صلة

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific

The regioselectivity of epoxide opening can depend on the conditions

Unsymmetrical bromonium ions open regioselectively

Electrophilic addition to unsymmetrical alkenes is regioselective

Alkenes react with bromine

Effects of substituents on CH2 groups

Effects of functional groups

Identifying compounds from nature