Nomenclature and Isomerism in Organic Chemistry

Question 1

Naming organic compounds: be able to apply IUPAC rules for nomenclature not only to the simple organic compounds, limited to chains with up to 6 carbon atoms, met at AS, but also to benzene and the functional groups listed in this unit.

How do you name stuff (excluding alkenes and alcohols)?

Answer 1

1. Count the carbons in the longest continous chain, giving you the stem/start (prefix). Longest chain may be bent.
   1 Mother Meth-
   2 Eats Eth-
   3 Peanut Prop-
   4 Butter But-
   5 Producing Pent-
   6 Heroin Hex-
2. Identify the main functional group; gives you the end (suffix). [haloalkanes, alcohols (suffix -ol when only group) and amines (suffix -amine when only group) give prefixes instead]
   -PICTURE

Cyclo- used to indicate a ring.
3. Number the carbons in the longest chain so that the carbon with the main functional group attached has the lowest possible number. Pick the one with most branches/side chains if more than one longest chain.
4. Write the carbon number that the functional group is on before the suffix;
2-iodo.., 3-ethyl..
5. Side-chains/less important functional groups added as prefixes at the start of the name. Alphabetise, with number of the C atom each one is attached to.
6. Use di-, tri- or tetra- if more than one identical functional group/side chain, before its name.

Question 2

How do you name alkenes?

Answer 2

1. Count carbon of longest change, identify stem. (meth, eth, prop etc.)
2. Number carbons so that the double bond has the lowest possible number.
3. Stem-number of carbon of C=C bond-ene.

If the alkene has two double bonds, the suffix (end) becomes -diene, stem gets an extra ‘a’ (buta-, penta-) and numbers are written first, not in the middle.

Question 3

How do you name alcohols?

Answer 3

If alcohol is only group (not including alkyl side chains):

1. Count the carbons to get the stem.
2. Count which carbon the -OH group is bonded to; lowest number carbon.
3. Primary, secondary or tertiary:
   Propan-1-ol, Propan-2-ol,
   2-methylpropan-2-ol.

Question 4

How do you draw the display formula of a molecule from its name?

Answer 4

1. Identify the root, the stem; carbon structure.
   - ane; alkane, no double bonds.
   - ene; alkene, double bond.
   - yne; alkyne, triple bond.
2. Identify functional groups; (usually) the end of the name. Prefixes/suffixes tell you the functional group.

Side chains show by a prefix; methyl, ethyl etc; alkyl groups; name tells you the number of carbons. C_nH_2-1

3. Draw.

Question 5

What is structural isomerism?

Answer 5

Same molecular formula but different structural formula; atoms are arranged/bonded in a completely different order.

Types:

Chain isomers; different arrangements of the carbon skeleton; branching vs. straight chain.
E.g. butane and methylpropane are chain isomers.

Positional isomers; basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton; functional group is attached to a different carbon atom.
E.g. 1-chlorobutane and 2-chlorobutane are position isomers.

Functional group isomers; have the same atoms arranged into completely different functional groups; belong to different family of compounds, different homologous series.
E.g. propanal, propanone and cyclopropanol are all functional group isomers of C₃H₆O.

If you are asked to draw the structural isomers from a given molecular formula, don’t forget to think about all the possibilities. Can you branch the carbon chain? Can you move a group around on that chain? Is it possible to make more than one type of compound?

Question 6

What is stereoisomerism?
State two forms.

Answer 6

Compounds have the same structural formula but a different arrangement in space; same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space.

E-Z isomerism
Stereoisomers happen when each C=C bonded carbon atom has two different atoms/groups attached to it. Then you get an ‘E-isomer’ (diagnolly opposite) and a ‘Z-isomer’ (zusammen; together, same side of double bond).

There is no rotation around a double bond; Z- and E-isomers are seperate compounds and not easily converted to one another.

Optical isomerism
Arises when there are four different substituents attached to one carbon atom; results in two isomers that are mirror images of one another, but not identical. Like your shoes.

Question 7

Optical isomerism; tell me more.

Answer 7

Arises when there are four different substituent groups attached to one carbon atom; a chiral/asymmetric carbon atom. Results in two isomers that are mirror images of one another, but not identical. Like your shoes.

Possible to arrange the groups in two different ways as mentioned above; enantiomers/optical isomers.

Enantiomers are mirror images; can’t be superimposed.
(if they can be superimposed, they’re achiral; there’s no optical isomerism)

These pairs are called optical isomers because they differ in the way they rotate the plane of polarisation of polarised light; one enantiomer rotates it in a clockwise direction, the other in an anticlockwise direction.

Chiral centre located by looking for the carbon atom with four different groups attached; though may be attached to two other carbons, these carbons may from parts of different groups, thus counting them as another group.

Question 8

What is a racemate/racemic mixture?
How are they formed?

Answer 8

A racemate/racemic mixture contains equal quantities of each eantiomer of an optically active compound; a 50:50 mixture of two optical isomers. They are not optically active as the two enatiomers cancel each other out.

A racemate is formed when two achiral things are reacted together, getting a racemic mixture of a chiral product; when the two molecules react, there’s an equal chance of forming each of the enantiomers.

You can modify a reaction to produce a single enantiomer using chemical methods, but it’s difficult and expensive.

Question 9

What is optical isomerism’s relevance to drugs?

Answer 9

Drug action may be determined by the stereochemistry of the molecule and that different optical isomers may have very different effects.

A drug must be exactly the right shape to fit into the correct active site; only one enantiomer will do. The other enantiomer could fit into a different enzyme, and might cause harmful side-effects or have no effect at all.

So if a drug happens to be chiral, it must be made so that it only contains one enantiomer. E.g. Thalidomide; one enantiomer relieved morning sickness, the other caused mutations in babies, being born with malformed limbs; birth defects.

In some cases, one enantiomer is an effective drug and the other is inactive:

• Can seperate the two isomers; but may be difficult or expesive as they have similar properties
• Can sell the mixture as a drug; but wasteful as half is inactive.
• Can design an alternative synthesis of the drug that only makes the required isomer.

Ibruprofen is a mixture of two optical isomers; mostly made and sold as a racemate.