Organic Chemistry Basics: Hybridisation and Functional Groups

Why does carbon get its own branch of chemistry?

Organic chemistry is the study of carbon compounds, and carbon earns that attention for two reasons.

First, carbon commonly forms four bonds. Oxygen forms two and nitrogen three, so carbon offers more connection points. Second, carbon bonds to itself well, which allows long chains, branches and rings. Add hydrogen, oxygen, nitrogen and a few other elements, and the number of possible molecules is enormous.

Carbon–carbon and carbon–hydrogen bonds are also strong and not very polar. That makes the backbone of a molecule stable, which is part of why the chemistry of living things can be built on it.

Hybridisation: counting what is attached

You do not need quantum mechanics to predict the shape around a carbon atom. You need one count: the number of atoms bonded to it, plus any lone pairs. Chemists call this the steric number.

• 4 attached groups: sp³, tetrahedral, about 109.5° between bonds.
• 3 attached groups: sp², trigonal planar, about 120°.
• 2 attached groups: sp, linear, 180°.

The reason is the same repulsion idea used for small molecules: electron regions spread as far apart as they can. A double bond counts as one region, which is why a carbon in a C=C double bond has three attached groups and is flat.

What this looks like in real molecules

Methane's carbon is sp³ and fully 3D. Ethene's carbons are sp² and the whole group of atoms lies in one plane. Ethyne's carbons are sp and the molecule is a straight line. Once you can read the count, you can sketch the shape of an unfamiliar molecule before looking at a model.

Drawing molecules the way chemists do

Writing out every atom gets cluttered fast, so chemists use skeletal structures. Each line is a bond, and each corner or line-end is a carbon atom. Hydrogens on carbon are left out, and you fill them in mentally so that every carbon has four bonds in total. Atoms other than carbon and hydrogen are labelled, along with any hydrogens attached to them.

A zig-zag line with six corners is a six-carbon chain. A hexagon with alternating double bonds is a ring of six sp² carbons. The shorthand feels odd for a day or two and then becomes faster than any alternative.

One caution: skeletal drawings are flat, but molecules are not. Wedges and dashes show which bonds point toward or away from you, and rotating a 3D model is the best way to learn to read them.

Functional groups: where the action is

A long carbon chain is chemically fairly dull. The reactions happen at functional groups, small arrangements of atoms with reliable behaviour. Learn to spot them and you can guess how a molecule will act.

Common examples:

• Alcohol (–OH): can hydrogen bond, so small alcohols mix with water.
• Carbonyl (C=O): a carbon double bonded to oxygen, found in aldehydes, ketones, acids and esters.
• Carboxylic acid (–COOH): can release a hydrogen ion.
• Amine (–NH₂): a nitrogen that can act as a base.
• Alkene (C=C): an electron-rich double bond that is ready to react.

The same group behaves in a similar way wherever it appears. That is what turns thousands of molecules into a few dozen patterns.

Polarity and intermolecular forces

A bond between different elements is polar when one atom attracts the shared electrons more strongly. Carbon and hydrogen are nearly equal in this respect, so C–H is only weakly polar. C–O and C–N are noticeably polar, and C=O is strongly so.

That has practical consequences. The carbon of a carbonyl ends up electron poor, which makes it a target for electron-rich species. Polar molecules attract each other more strongly, so they usually boil at higher temperatures than non-polar molecules of similar size. And as a rough rule, like dissolves like: polar substances dissolve polar ones, non-polar mix with non-polar.

Acids, bases and stability

Most organic reactions come down to a pair of electrons moving from a rich place to a poor place. Acid–base chemistry is the cleanest version. The measure is pKa. Lower pKa means stronger acid, and a drop of one unit makes the acid ten times stronger.

The useful question is not "is this acidic?" but "how comfortable is the leftover anion?" An acid that gives up a hydrogen ion leaves behind a negative charge, and the more that charge can be spread out or held by an electronegative atom, the stronger the acid. This stability thinking comes back again and again in later topics.

Mistakes to avoid

• Memorising shapes instead of counting. Count attached groups and lone pairs, then read the shape off.
• Counting a double bond as two groups. For hybridisation it is one.
• Forgetting hidden hydrogens in skeletal drawings.
• Assuming all bonds to carbon are polar. C–H is nearly non-polar.
• Reading a flat drawing as a flat molecule.

How to explore this in the lab

The Organic Chemistry lab is a simplified teaching model of real molecules, with rotation so you can see what flat drawings hide. Suggested order through the free lessons:

1. Why carbon for the big picture.
2. sp³, sp² and sp while rotating each geometry.
3. Skeletal structures to practise the shorthand.
4. Functional groups then Polarity and forces.

If atoms and bonds still feel shaky, the Chemistry Lab covers them from the beginning.