Organic Chemistry lessons
Carbon's family
An alkane chain grows one CH2 unit at a time from methane to octane, with its formula, mass and boiling point plotted against room temperature.
Organic chemistry is the chemistry of carbon compounds. Fuels, plastics, medicines and the molecules in your body are all organic.
Carbon atoms join into chains. Adding one \text{CH}_{2} unit at a time makes a family of molecules whose properties change step by step.
This course shows how to draw, name and sort these molecules. It ends with the reactions that burn them and join them into plastics.
Why carbon is special
A carbon atom has four electrons in its outer shell. It shares each one in a covalent bond, so carbon always makes four bonds.
Hydrogen always makes one bond and oxygen makes two. Carbon atoms also bond strongly to each other, so they can build chains, branches and rings.
Carbon skeletons built one at a time: one carbon, a chain, a branch and a ring, with hydrogens filling every spare bond.
A molecule made only of carbon and hydrogen is a hydrocarbon. Build the carbon skeleton first, then add hydrogens until every carbon has four bonds.
Practice: Four bonds each
Tap the carbon in a displayed formula that does not have four bonds.
Four ways to draw a molecule
A molecular formula just counts the atoms, like \text{C}_{4}\text{H}_{10}\text{O}. A structural formula writes each carbon with the atoms on it, like \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{OH}.
A displayed formula draws every atom and every bond. A skeletal formula draws just the carbon zigzag, plus any atoms that are not carbon or hydrogen.
Butan-1-ol as a displayed, structural, molecular and skeletal formula, the drawing morphing from one to the next.
Reading skeletal formulas
In a skeletal formula, every corner and every line end is a carbon atom. Each carbon has enough hydrogens to make four bonds, but they are left out.
A double line is a double bond, and other atoms are written in, like \text{OH}. Rings are easy to draw this way: benzene, \text{C}_{6}\text{H}_{6}, is a hexagon of six carbons.
A 3D benzene molecule: the flat ring of six carbons, one hydrogen on each, and the ring of shared electrons.
Practice: Match the drawings
Drag each molecular formula onto its displayed or skeletal formula.
Alkanes
Alkanes are hydrocarbons with only single bonds. Every carbon holds as many hydrogens as it can, so alkanes are saturated.
General formula of the alkanes: \text{C}_n\text{H}_{2n+2}
Every carbon in the chain carries two hydrogens, and the two end carbons carry one more each. So an alkane with n carbons has 2n+2 hydrogens.
Their names start meth, eth, prop, but, pent, hex, hept and oct for 1 to 8 carbons. A family where each member has one more \text{CH}_{2} than the last is a homologous series.
Practice: Write the formula
Type the molecular formula of an alkane from its name or its number of carbons.
Why longer chains boil higher
Molecules attract each other with weak intermolecular forces. To boil a liquid, its molecules need enough energy to pull apart.
Longer chains touch each other along more of their length, so the attraction between them is stronger. Each alkane boils at a higher temperature than the one before.
Two chains side by side are warmed until they pull apart; longer chains hold on until a higher temperature.
Boiling never breaks the bonds inside a molecule. It only overcomes the attractions between molecules.
Practice: Order by boiling point
Put four alkanes in order of boiling point, lowest first.
Alkenes and the double bond
Alkenes have a carbon to carbon double bond, \text{C}=\text{C}. The two carbons share two pairs of electrons instead of one.
General formula of the alkenes: \text{C}_n\text{H}_{2n}
An alkene has two fewer hydrogens than the alkane with the same number of carbons, so it is unsaturated. The names end in ene, starting with ethene, \text{C}_{2}\text{H}_{4}.
3D methane and ethene: four single bonds point to the corners of a tetrahedron, while a double bond holds its atoms flat.
Four single bonds point to the corners of a tetrahedron, 109.5° apart. Around a double bond the atoms lie flat, about 120° apart.
The bromine water test
Bromine water is orange. Shake it with an alkene and it turns colourless.
The double bond opens and one bromine atom joins each carbon. Alkanes have no double bond to open, so the bromine water stays orange.
Bromine water shaken with hexane and with hexene: the alkene uses up the bromine and the orange fades.
Practice: Will it decolourise?
Sort names and formulas by what they do to bromine water.
Functional groups
A functional group is the part of a molecule that decides how it reacts. Molecules with the same group share a name ending and a general formula.
Alcohols carry an \text{OH} group and end in ol, like ethanol. Carboxylic acids carry a \text{COOH} group and end in oic acid, like the ethanoic acid in vinegar.
The same two-carbon backbone with different functional groups, each with its name ending and what it does.
An alcohol and a carboxylic acid react to make an ester, which often smells fruity. Amines carry an \text{NH}_{2} group and often smell fishy.
Practice: Name the family
Pick the family of a molecule from its displayed or skeletal formula.
Naming step by step
A name is built from parts. The stem counts the carbons in the longest chain, and the ending names the family: ane, ene, ol or oic acid.
Number the chain from the end that gives the group the lowest number, and put that number before the ending. Branches go at the front with their own numbers, like 2-methyl.
Three molecules named step by step: longest chain, family, numbering from the right end, branches, then the full name.
Practice: Build the name
Build the name of a random alkane, alkene or alcohol from its branch, stem, number and ending.
Structural isomers
Structural isomers have the same molecular formula, but their atoms are joined in a different order. Pentane, \text{C}_{5}\text{H}_{12}, has three.
Branched isomers are more compact, so they touch each other less. Their intermolecular forces are weaker, so they boil at lower temperatures.
The three isomers of C5H12 with their shapes and boiling points.
Bending a chain does not make an isomer. If every atom is joined to the same neighbours, it is the same molecule.
Practice: Pick the isomers
Tap every skeletal formula that has a given molecular formula.
Burning hydrocarbons
In plenty of oxygen, a hydrocarbon burns to carbon dioxide and water. This complete combustion releases the energy we get from fuels.
Atoms are never made or destroyed, so a balanced equation has the same atoms on both sides. Balance carbon first, then hydrogen, then oxygen.
Combustion equations balanced step by step, with the atoms on each side counted as they go.
Without enough oxygen, incomplete combustion makes toxic carbon monoxide or sooty carbon instead.
Practice: Balance the equation
Type the four smallest whole-number coefficients that balance a combustion equation.
Addition polymers
A polymer is a huge molecule made by joining thousands of small monomers. Alkenes join when their double bonds open up, which is called addition polymerisation.
Ethene makes poly(ethene), the plastic in carrier bags. The repeat unit is drawn in brackets with an n for the many times it repeats.
Monomers open their double bonds and link into a chain, then the repeat unit is boxed in brackets.
Practice: Find the repeat unit
Pick the repeat unit made from a monomer, or the monomer behind a repeat unit.