Elements of Unsaturation

Elements of Unsaturation

(1) Elements of Unsaturation in Hydrocarbons

Alkenes are said to be unsaturated because they are capable of adding hydrogen in the presence of a catalyst.

– The product, an alkane, is called saturated because it cannot react with any more hydrogen.

– The presence of a pi bond of an alkene (or an alkyne) or the ring of a cyclic compound decreases the number of hydrogen atoms in a molecular formula.

– These structural features are called elements of unsaturation.

– Each element of unsaturation corresponds to two fewer hydrogen atoms than in the “saturated” formula.

Elements of Unsaturation

– Consider, for example, the formula C4H8.

– A saturated alkane would have a CnH(2n+2) formula, or C4H10.

– The formula C4H8 is missing two hydrogen atoms, so it has one element of unsaturation, either a pi bond or a ring.

– There are five constitutional isomers of formula C4H8:

Elements of Unsaturation

– When you need a structure for a particular molecular formula, it helps to find the number of elements of unsaturation.

– Calculate the maximum number of hydrogen atoms from the saturated formula, CnH(2n+2) and see how many are missing.

– The number of elements of unsaturation is simply half the number of missing hydrogens.

– This simple calculation allows you to consider possible structures quickly, without always having to check for the correct molecular formula.

(2) Elements of Unsaturation with Heteroatoms

– Heteroatoms (hetero, “different”) are any atoms other than carbon and hydrogen.

– The rule for calculating elements of unsaturation in hydrocarbons can be extended to include heteroatoms.

– Let’s consider how the addition of a heteroatom affects the number of hydrogen atoms in the formula.


– Halogens simply substitute for hydrogen atoms in the molecular formula.

– The formula C2H6 is saturated, so the formula C2H4F2 is also saturated.

– C4H8 has one element of unsaturation, and C4H5Br3 also has one element of unsaturation.

– In calculating the number of elements of unsaturation, simply count halogens as hydrogen atoms.

Elements of Unsaturation


– An oxygen atom can be added to the chain (or added to a C-H bond to  make a C-OH group) without changing the number of hydrogen atoms or carbon atoms.

– In calculating the number of elements of unsaturation, ignore the oxygen atoms.

Elements of Unsaturation


– A nitrogen atom can take the place of a carbon atom in the chain, but nitrogen is trivalent, having only one additional hydrogen atom, compared with two hydrogens for each additional carbon atom.

– In computing the elements of unsaturation, count nitrogen as half a carbon atom.

Elements of Unsaturation

– The formula C4H9N is like a formula with 4.5 carbon atoms, with saturated formula C4.5H9+2

– The formula C4H9N has one element of unsaturation, because it is two hydrogen atoms short of the saturated formula.

Solved Problem

Draw at least four compounds of formula C4H6NOCl.


– Counting the nitrogen as 1/2 carbon, ignoring the oxygen, and counting chlorine as a hydrogen shows the formula is equivalent to C4.5H7.

– The saturated formula for 4.5 carbon atoms is C4.5H11, so C4H6NOCl  has two elements of unsaturation.

– These could be two double bonds, two rings, one triple bond, or a ring and a double bond.

– There are many possibilities, four of which are listed here.

Elements of Unsaturation

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