Electronegativity – polar bonds in organic compounds

What is electronegativity?

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is given a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.

What happens if two atoms of equal electronegativity bond together?

The most obvious example of this is the bond between two carbon atoms. Both atoms will attract the bonding pair to exactly the same extent. That means that on average the electron pair will be found half way between the two nuclei, and you could draw a picture of the bond like this:

It is important to realise that this is an average picture. The electrons are actually in a sigma orbital, and are moving constantly within that orbital.

Help!  A sigma orbital is a molecular orbital formed by end-to-end overlap between two atomic orbitals. 

The carbon-fluorine bond

Fluorine is much more electronegative than carbon. The actual values on the Pauling scale are

That means that fluorine attracts the bonding pair much more strongly than carbon does. The bond – on average – will look like this:

Why is fluorine more electronegative than carbon?

A simple dots-and-crosses diagram of a C-F bond is perfectly adequate to explain it.

The bonding pair is in the second energy level of both carbon and fluorine, so in the absence of any other effect, the distance of the pair from both nuclei would be the same.

The electron pair is shielded from the full force of both nuclei by the 1s electrons – again there is nothing to pull it closer to one atom than the other.

BUT, the fluorine nucleus has 9 protons whereas the carbon nucleus has only 6.

Allowing for the shielding effect of the 1s electrons, the bonding pair feels a net pull of about 4+ from the carbon, but about 7+ from the fluorine. It is this extra nuclear charge which pulls the bonding pair (on average) closer to the fluorine than the carbon.

Help!  You have to imagine what the bonding pair “sees” if it looks in towards the nucleus. In the carbon case, it sees 6 positive protons, and 2 negative electrons. That means that there will be a net pull from the carbon of about 4+. The shielding wouldn’t actually be quite as high as 2-, because the 1s electrons spend some of their time on the far side of the carbon nucleus – and so aren’t always between the bonding pair and the nucleus.

Incidentally, thinking about electrons looking towards the nucleus may be helpful in picturing what is going on, but avoid using terms like this in exams.

The carbon-chlorine bond

The electronegativities are:

The bonding pair of electrons will be dragged towards the chlorine but not as much as in the fluorine case. Chlorine isn’t as electronegative as fluorine.

Why isn’t chlorine as electronegative as fluorine?

Chlorine is a bigger atom than fluorine.

fluorine: 1s²2s²2p_x²2p_y²2p_z¹

chlorine: 1s²2s²2p_x²2p_y²2p_z²3s²3p_x²3p_y²3p_z¹

In the chlorine case, the bonding pair will be shielded by all the 1-level and 2-level electrons. The 17 protons on the nucleus will be shielded by a total of 10 electrons, giving a net pull from the chlorine of about 7+.

That is the same as the pull from the fluorine, but with chlorine the bonding pair starts off further away from the nucleus because it is in the 3-level. Since it is further away, it feels the pull from the nucleus less strongly.

Bond polarity and inductive effects

Polar bonds

Think about the carbon-fluorine bond again. Because the bonding pair is pulled towards the fluorine end of the bond, that end is left rather more negative than it would otherwise be. The carbon end is left rather short of electrons and so becomes slightly positive.

The symbols + and – mean “slightly positive” and “slightly negative”. You read + as “delta plus” or “delta positive”.

We describe a bond having one end slightly positive and the other end slightly negative as being polar.

Inductive effects

An atom like fluorine which can pull the bonding pair away from the atom it is attached to is said to have a negative inductive effect.

Most atoms that you will come across have a negative inductive effect when they are attached to a carbon atom, because they are mostly more electronegative than carbon.

You will come across some groups of atoms which have a slight positive inductive effect – they “push” electrons towards the carbon they are attached to, making it slightly negative.

Inductive effects are sometimes given symbols: -I (a negative inductive effect) and +I (a positive inductive effect).

Note:  You should be aware of terms like “negative inductive effect”, but don’t get bogged down in them. Provided that you understand what happens when electronegative atoms like fluorine or chlorine are attached to carbon atoms in terms of the polarity of the bonds, that’s really all you need for most purposes.

Some important examples of polar bonds

Hydrogen bromide (and other hydrogen halides)

Bromine (and the other halogens) are all more electronegative than hydrogen, and so all the hydrogen halides have polar bonds with the hydrogen end slightly positive and the halogen end slightly negative.

Help!  Halogen: a member of group VII of the Periodic Table – fluorine, chlorine, bromine and iodine.

Halide: a compound of one of these – e.g. hydrogen chloride, hydrogen bromide, etc.

The polarity of these molecules is important in their reactions with alkenes.

The carbon-bromine bond in halogenoalkanes

Note:  You may come across halogenoalkanes under the names “haloalkanes” or “alkyl halides”.

Bromine is more electronegative than carbon and so the bond is polarised in the way that we have already described with C-F and C-Cl.

The polarity of the carbon-halogen bonds is important in the reactions of the halogenoalkanes.

The carbon-oxygen double bond

An orbital model of the C=O bond in methanal, HCHO, looks like this:

The very electronegative oxygen atom pulls both bonding pairs towards itself – in the sigma bond and the pi bond. That leaves the oxygen fairly negative and the carbon fairly positive.