4.1.3 (a,b) Properties of Alkenes

Syllabus

(a) Alkenes as unsaturated hydrocarbons containing a C=C bond comprising a π-bond (sideways overlap of adjacent p-orbitals above and below the bonding C atoms) and a σ-bond (overlap of orbitals directly between the bonding atoms) (see also 4.1.2 a); restricted rotation of the π-bond

{Hybridisation is not required.}

{Use of model of orbital overlap to explain covalent bonding in organic compounds.}

(b) explanation of the trigonal planar shape and bond angle around each carbon in the C=C of Alkenes in terms of electron pair repulsion (see also 2.2.2 g–h, 4.1.2 b)

What does this mean?

The Basics

Alkenes are Hydrocarbons because they contain only Carbon and Hydrogen atoms.

They are unsaturated because they contain a C=C double-bond - this is what to write in an exam.

Alternatively, they are unsaturated because the Carbon atoms are not bonded to the maximum number of atoms - but don't write this in the exam.

The naming of Alkenes follows the Alkane model:- Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec.

Although you can't have Methene because an Alkene must contain a C=C bond and Meth implies only one C atom.

Their general formula is C_nH_2n.

There are no possible isomers of Alkenes until we get to Butene.

Because the double bond can be at the end of the molecule or in the middle we can have But-1-ene (where the lowest numbered Carbon atom touched by the double bond is No 1) and But-2-ene (where the lowest numbered Carbon atom touched by the double bond is No 2)

Looking at the bond enthalpy we can see that the difference between C-C and C+C is only 260 kJ/mol compared to the 350 kJ/mol for the sigma bond on its own.

Adding another pi bond to make a treble bond adds a similar amount of extra strength (230 kJ/mol).

When s-orbitals overlap they can freely rotate without ever breaking the overlap - which would require some energy.

Even if p, d or f orbitals overlapped head-on they could rotate without losing contact.

But in a pi bond the p-orbitals are overlapping sideways - above and below the plane of the sigma bond.

Any rotation would mean that contact is lost and the bond has broken.

But since breaking bonds needs energy pi bonds don't rotate in normal circumstances.

As a consequence many Alkenes have geometrical or stereo isomers (see later).

Bond Geometry

VSEPR tells us that double-bonds do not repel any more than single bonds.

So a Carbon atom in a double bond effectively has three bonds and no lone-pairs.

The furthest apart three bonds can get is 120^o - which is trigonal planar.

So all alkenes are trigonal planar about the Carbon atoms in the C=C bond.

Although they are very likely to have other Carbon atoms in a tetrahedral arrangement because they have 4 bonds.

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