Naming Organic Compounds

In short: Organic compounds are named from their longest carbon chain (meth, eth, prop, but and so on) and an ending for their functional group, like -ane, -ene, -ol or -oic acid. Number the chain so the functional group gets the lowest number, and list any branches in alphabetical order.

Organic chemistry and carbon

Organic chemistry is the chemistry of carbon compounds (apart from simple ones like $C O_{2}$ and carbonates). Carbon is special because each carbon atom:

makes four covalent bonds,
can bond to other carbon atoms in long chains and rings,
can form single, double and triple bonds.

Homologous series

A homologous series is a family of compounds that:

have the same functional group,
have the same general formula,
differ from the next member by $C H_{2}$ ,
show a steady trend in physical properties (like boiling point) and similar chemical properties.

The prefix: chain length

The longest carbon chain gives the stem of the name.

Carbons	Stem	Carbons	Stem
1	meth-	5	pent-
2	eth-	6	hex-
3	prop-	7	hept-
4	but-	8	oct-

The suffix: functional groups

Series	Functional group	Ending	Example
Alkane	C−C only	-ane	propane $C H_{3} C H_{2} C H_{3}$
Alkene	C=C	-ene	propene $C H_{2} = CHC H_{3}$
Alkyne	C≡C	-yne	propyne
Haloalkane	−F, −Cl, −Br, −I	prefix fluoro-, chloro-, bromo-, iodo-	1-chloropropane
Alcohol	−OH (hydroxyl)	-ol	propan-1-ol
Aldehyde	−CHO (at the end of the chain)	-al	propanal
Ketone	C=O (in the middle of the chain)	-one	propanone
Carboxylic acid	−COOH (carboxyl)	-oic acid	propanoic acid
Amine	−NH₂ (amino)	-amine	propan-1-amine
Amide	−CONH₂	-amide	propanamide
Ester	−COO−	-yl -oate	methyl ethanoate

IUPAC naming rules

Find the longest carbon chain that contains the functional group. This gives the stem.
Number the chain from the end that gives the functional group the lowest number. If there is no functional group, give the branches the lowest numbers.
Name the branches (methyl $- C H_{3}$ , ethyl $- C_{2} H_{5}$ ) and halogens as prefixes, with their positions.
List prefixes in alphabetical order (bromo before chloro before methyl). Use di-, tri-, tetra- for repeats, but ignore them when sorting.
Use commas between numbers and hyphens between numbers and words.

Esters are named from the alcohol part (as an "-yl" word) then the acid part (as an "-oate" word). Ethanol + ethanoic acid gives ethyl ethanoate, $C H_{3} COOC H_{2} C H_{3}$ .

Amides with a group on the nitrogen use "N-": N-methylpropanamide has a methyl group on the N.

Drawing organic molecules

Type	Shows	Example (propan-1-ol)
Molecular formula	Number of each atom	$C_{3} H_{8} O$
Condensed structural	Groups in order	$C H_{3} C H_{2} C H_{2} OH$
Full structural	Every atom and bond	All C−H, C−C, C−O, O−H bonds drawn
Skeletal	Zigzag line; each corner or end is a carbon; H on carbon not shown	A zigzag of 3 points with OH on the end

Structural isomers

Structural isomers have the same molecular formula but different structures.

Chain isomers: the carbon chain is branched differently. Butane and 2-methylpropane ( $C_{4} H_{10}$ ).
Position isomers: the functional group is in a different place. Propan-1-ol and propan-2-ol.
Functional group isomers: a different functional group altogether. Propanal and propanone ( $C_{3} H_{6} O$ ), or propanoic acid and methyl ethanoate ( $C_{3} H_{6} O_{2}$ ).

Formula	Number of structural isomers
$C_{4} H_{10}$	2
$C_{5} H_{12}$	3
$C_{6} H_{14}$	5
$C_{4} H_{10} O$ (alcohols only)	4

Naming organic compounds

Carbon

Four bonds
Chains and rings

Homologous series

Same functional group
Differ by CH2

Name parts

Stem: chain length
Ending: functional group
Prefixes: branches, halogens

Rules

Longest chain
Lowest numbers
Alphabetical prefixes

Formulas

Molecular, condensed
Structural, skeletal

Isomers

Chain
Position
Functional group

Year 11Counting isomers carefully

To count isomers, draw them in order and check for repeats. A molecule flipped or rotated is the same molecule.

$C_{5} H_{12}$ : pentane (straight), 2-methylbutane, 2,2-dimethylpropane. "3-methylbutane" is just 2-methylbutane numbered from the wrong end, so it is not new. Three isomers.

$C_{3} H_{6} C l_{2}$ : 1,1-, 1,2-, 1,3- and 2,2-dichloropropane. Four isomers.

$C_{4} H_{10} O$ alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. Four isomers.

$C_{2} H_{4} B r_{2}$ : 1,1- and 1,2-dibromoethane. Two isomers.

$C_{3} H_{6} BrCl$ : two different halogens give more choices: 1-bromo-1-chloro, 1-bromo-2-chloro, 1-bromo-3-chloro, 2-bromo-1-chloro and 2-bromo-2-chloropropane. Five isomers.

$C_{6} H_{14}$ : hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane. Five chain isomers.

Counting substitution products. Count the different kinds of hydrogen. In 2,2-dimethylbutane, $(C H_{3})_{3} CC H_{2} C H_{3}$ , there are three: the nine H on the three matching methyls, the $C H_{2}$ , and the end $C H_{3}$ . So three monochloro products.

Year 12Harder names

Two functional groups. The higher-priority group gets the ending; the other becomes a prefix. Priority: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene. So $C H_{3} CH (OH) C H_{2} COOH$ is 3-hydroxybutanoic acid, and $C H_{3} COC H_{2} C H_{2} CH (OH) C H_{3}$ is 5-hydroxyhexan-2-one.

Alkenes with branches. Number to give the double bond the lowest number first: $C H_{2} = C (C H_{3}) CH (C H_{3}) C H_{3}$ is 2,3-dimethylbut-1-ene.

Cis and trans. But-2-ene has two forms: with both $C H_{3}$ groups on the same side of the double bond (cis) or opposite sides (trans). The C=C bond cannot rotate, so they are different molecules. These are stereoisomers, not structural isomers.

Halogens with a main group. Number from the end that gives the main group the lowest number, then list halogens and branches alphabetically. In $C H_{3} CHBrC H_{2} C (C H_{3}) IC H_{2} CHO$ the aldehyde carbon is C1, so the name is 5-bromo-3-iodo-3-methylhexanal.

Quick summary

A homologous series has one functional group, one general formula, and members that differ by $C H_{2}$ .
Stems: meth, eth, prop, but, pent, hex, hept, oct.
Endings: -ane, -ene, -yne, -ol, -al, -one, -oic acid, -amine, -amide, -yl -oate.
Longest chain, lowest numbers for the functional group, prefixes in alphabetical order.
Structural isomers: same molecular formula, different structure (chain, position or functional group).

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How many covalent bonds does a carbon atom usually make?
1. A 2
2. B 3
3. C 4
4. D 6
Show answer
C: 4
Carbon has four outer electrons to share.
The stem for a 5-carbon chain is:
1. A but-
2. B pent-
3. C hex-
4. D prop-
Show answer
B: pent-
meth 1, eth 2, prop 3, but 4, pent 5.
Members of a homologous series differ by:
1. A $C H_{2}$
2. B $C H_{3}$
3. C One carbon atom only
4. D $H_{2} O$
Show answer
A: $C H_{2}$
Each member has one more $C H_{2}$ unit.

Organic chemistry and carbon

Homologous series

The prefix: chain length

The suffix: functional groups

IUPAC naming rules

Drawing organic molecules

Structural isomers

Quick summary

Quiz

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