Organic nomenclature

What is it? The systematic naming of an organic compound generally requires the identification and naming of a parent structure. This name may then be modified by prefixes, infixes, and, in the case of a parent hydride, suffixes, which give the precise structural changes required to generate the actual compound from the parent structure.

Naming straight-chain alkanes Naming of straight chain alkanes (alkanes that do not branch) is a straightforward process. To give an alkane a name, a prefix indicating the number of carbons in the molecule is added to the suffix ane, identifying both the kind of molecule (an alkane) and how many carbons the molecule has (the prefix). The name pentane, for example, tells you that the molecule is an alkane (thus the ane ending) and that it has five carbons (pent indicates five).

Prefixes for alkanes that have 1-4 carbons are rooted historically. These are methane, ethane, propane, and butane, respectively. On the other hand, for 5 carbons and up a prefix derived from greek is given. (An easy way to remember the first four names is the anagram Mary eats peanut butter, standing for methane, ethane, propane, butane). Learning the prefixes for up to twelve carbons is a good idea.

Number of Carbons Prefix Structure 1 Methane CH4 2 Ethane CH3CH3 3 Propane CH3CH2CH3 4 Butane CH3(CH2)2CH3 5 Pentane CH3(CH2)3CH3 6 Hexane CH3(CH2)4CH3 7 Heptane CH3(CH2)5CH3 8 Octane CH3(CH2)6CH3 9 Nonane CH3(CH2)7CH3 10 Decane CH3(CH2)8CH3 11 Undecane CH3(CH2)9CH3 12 Dodecane CH3(CH2)10CH3

Naming branched alkanes The nomenclature becomes more complex if the alkane branches. In such a case, there are several rules that you must follow to give the alkane the correct name. Find the longest chain of carbons in the molecule. The number of carbons in the longest chain becomes the parent name. After finding the parent chain, you number the parent chain starting with the end nearest the first substituent (a substituent is any fragment that juts off the main chain).

Next, determine the names of all substituents Next, determine the names of all substituents. Substituents are named as if the piece were a separate molecule, except that the suffix of yl is used rather than ane. Thus, a two-carbon substituent would be an ethyl substituent (not an ethane substituent). Put the substituents in alphabetical order (ie. ethyl before methyl) in front of the parent name. Next, identify the positions of all substituents in the name by placing the carbon number where the substituent attaches to the parent chain in front of it. For example, 2-methylheptane indicates that a methyl substituent is attached to the number 2 carbon.

Try this one…   a)      Which can be drawn as:

Step 1 Find the longest carbon chain in the molecule. First, begin by finding the parent chain in the molecule--that is, the longest possible chain of connecting carbons. Note that the parent chain is not necessarily the chain that simply follows from left to right. For example, if you were to count the number of carbons directly from left to right in this molecule you would get 7 carbons. This is not the parent chain, however! If you start at the left and then count up where the molecule branches, you find that there are 8 carbons by taking this route. This is is the longest chain and thus the parent chain is octane.

Right! 8 carbon chain Wrong! 7 carbon chain

Step 2 Number the parent chain. The second step is to number the carbons in the parent chain starting at the end closest to the first substituent. It is important to number the molecule from the correct end (in other words, in this example do you number the alkane from right to left or left to right). Following this rule, on this molecule you number from right to left, as the 2-carbon substituent is closer to that end.

Right! correct numbering Wrong! incorrect numbering 1 2 3 4 6 5 7 8 8

Step 3 Name all the substituents. You then identify the names of the substituents. In this case, the only substituent is a 2 carbon group at the number 4 carbon. This is an ethyl group. 4

Step 4 Put the substituents in alphabetical order. The next step is to put the substituents in alphabetical order (ie. ethyl before methyl) but since there is only one substituent this is unnecessary.

Step 5 Locate the substituent on the parent change by giving it a number. Thus, the proper nomenclature of this alkane is 4-ethyloctane. Note that a dash is used to separate the number from the substituent.

Your turn… What is this? And the structural formula is? HEXANE

Your turn… 2-METHYL PENTANE What is this? And the structural formula is? 2-METHYL PENTANE

Your turn… 3-METHYL PENTANE What is this? And the structural formula is? 3-METHYL PENTANE

The prefixes di, tri, tetra etc The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when putting the substituents in alphabetical order. Number of same groups Prefix 1 mono 2 di 3 tri 4 tetra

Your turn… 2,2-dimethylbutane What is this? And the structural formula is? 2,2-dimethylbutane

Your turn… 2,3-dimethylbutane What is this? And the structural formula is? 2,3-dimethylbutane

D

C

C

D

A little harder… Butanol… C4H10O What about these 4 structures? How many C, H, O? And what are they all called?

A little harder… butan-1-ol 2-methyl-propan-1-ol butan-2-ol

Homework… Draw and name all the possible structures with the formula: C3H7Br (Br = bromo) C5H12 Draw the following compounds: 3-ethyl-2,4-dimethylheptane 2,2,4-trimethylpentane 4-ethyl-3,5-dimethylheptane