4.1.1 (c) Organic Vocabulary

Syllabus

(i) homologous series (a series of organic compounds having the same functional group but with each successive member differing by CH₂)

(ii) functional group (a group of atoms responsible for the characteristic reactions of a compound)

(iii) alkyl group (of formula CnH_2n+1)

(iv) aliphatic ( containing C & H joined together in straight or branched chains or non-aromatic rings)

(v) alicyclic (an aliphatic compound arranged in non-aromatic rings with or without side chains)

(vi) aromatic (a compound containing a Benzene ring)

(vii) saturated (C-C bonds only) & unsaturated (the presence of C=C, C≡C and aromatic rings)

{Definition required for homologous series only.}

{R may be used to represent alkyl groups, but also other fragments of organic compounds not involved in reactions.}

{The terms saturated and unsaturated will be used to indicate the presence of multiple carbon–carbon bonds as distinct from the wider term ‘degree of saturation’ used also for any multiple bonds and cyclic compounds.}

What does this mean?

Functional groups and homologous series

Using long words correctly makes you sound cleverer than your friends doing A level Drama!

Using them incorrectly loses you marks in exams.

You probably know about the Alkane homologous series.

We can see that for every extra Carbon atom we are adding another two Hydrogen atoms.

So part of the definition of an homologous series is that its successive members differ by -CH₂

The other part of the definition is that everything in an homologous series must have the same functional group.

The functional group is the atom or atoms in a molecule where (most) reactions happen.

As it happens Alkanes don't have a functional group - what they share is a lack of a functional group.

Alkenes , which you have also probably studied, do have a functional group.

They contain a C=C double bond.

This is where most Alkene reactions will happen.

But their formulae still change by CH₂ each time they get longer.

Alcohols contain the functional group -OH

So, again, this is where most alcohol reactions happen.

And, again, every alcohol is CH₂ longer than its predecessor.

Alkyl groups

The naming of all organic substances is based on the naming system for alkanes.

So the parts of a substance that are similar to alkanes are called alkyl groups.

But because they are bonded to another atom or group there will be one H atom missing from the equivalent alkane.

In other words, an alkyl group is a piece of a molecule with the general formula C_nH_2n+1, where n is some integer. For example, a methyl group (CH₃) is a fragment of a methane molecule (CH₄); n = 1 in this case. The -yl ending means "a fragment of an alkane formed by removing a hydrogen".

So Chloroethane (right) contains an ethyl group

Ethane would be C₂H₆ and the ethyl group is one H short and is C₂H₅

So, since alkanes have the general formula C_nH_2n+2 ; alkyl groups must be C_nH_2n+1

Saturated, unsaturated.

Saturated molecules only have C-C single bonds - so each Carbon atom is bonded to the maximum number of other atoms.

Un saturated molecules contain many C-C single bonds but will also include at least one C=C - so not every Carbon atom is bonded to the maximum number of other atoms.

A polyunsaturated fat would contain many C=C bonds.

The presence of any Carbon-Carbon bond other than just C-C bonds would stop an molecule being classed as saturated.

So Benzene, which is often shown containing a mysterious circle, can also be thought of as containing 3 C=C bonds and so isn't saturated.

And alkynes, like ethyne (below) are also unsaturated.

But carboxylic acids like Propanoic acid (below) are saturated despite containing a C=O bond because all the Carbon-Carbon bonds are single bonds.

Aromatic, alicyclic, aliphatic

Any substance that contains a benzene ring is said to be aromatic.

This doesn't necessarily mean that they will actually smell strongly.

And it also doesn't mean that all smelly substances contain benzene rings.

Its just an old word based on the fact that most benzene-ring containing substances known then had strong smells.

Aliphatic is a word that basically means "non-aromatic".

So all alkanes and alkenes and alcohols and carboxylic acids etc would be considered aliphatic if they are not attached a Benzene ring.

So even alkanes with lots of side-groups are aliphatic eg 2,2,3,3-tetramethyl butane

Molecules can still be aliphatic and be in a ring shape.

But the ring must not a benzene ring.

Cycloalkanes like cyclopentane and cyclohexane are aliphatic and ring shaped.

So are sometimes called Alicyclic compounds.

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