Grade12 AP Chemistry



Functional Group Priority

Each type of functional group is given a priority level for numbering of carbons as well as for forming the suffix of the main chain name. The more functional groups that we learn, the more we must worry about the priority of each functional group in a molecule with different groups. Here is a priority list which includes all of the functional groups that we will learn this semester. Highest 1) Carboxylic acids 2) Esters 3) Amides 4) Aldehydes 5) Ketones 6) Alcohols 7) Amines 8) Ethers 9) Alkenes 10) Alkynes 11) Halides 12) Hydrocarbon branches Lowest The priority list shows you which functional groups have priority for the lowest carbon number and which gets priority for forming the base name for the molecule. In other words, if you have a hydrocarbon with both a hydroxy group (C-OH) and an ether functional group (C-O-C), do you name the molecule as an ether with a hydroxy branch or do you name it as an alcohol with an ether branch? Looking at the chart above, since alcohols are higher on the list that ethers, the molecule would be named as an alcohol with an ether "branch." In addition, the carbons are numbered so that the OH functional group is attached to the carbon with the lowest possible number.

Ether Nomenclature

Ethers are hydrocarbons in which an oxygen is spliced into the carbon chain: C-C-C-C-O-C-C The two lone pairs on the oxygen force the molecule into a bent configuration: C-C-C-C-O \ C \ C There are two accepted ways for naming ethers. Each method names the molecule as if the two carbon chains were branches off of the oxygen. Method 1: naming the longer carbon chain with an alkoxy branch. 1) Add the suffix "oxy" to the name of the shorter of the two hydrocarbon chains. 2) This shorter chain name is written first. 3) The name of the longer chain with its normal alkane/alkene/alkyne name is written second. Examples C-C-O-C-C-C C-C-O-C C-C-C-C-O-C-C-C C-C-C-O-C-C-C-C-C | C ethoxy propane methoxy ethane propoxy butane 2-methylpropoxy pentane The first example would not be called propoxy ethane, because the oxy suffix goes on the shorter carbon chain name! Method 2: the molecule is called an ether with two alkyl branches The alkyl branches are listed in order of increasing main chain. length: ethyl propyl ether methyl ethyl ether propyl butyl ether isobutyl pentyl ether In this method, when the two carbon chains are the same length, the name is given once with a "di" prefix: C-C-C-O-C-C-C dipropyl ether Mixed Functional Groups: for general interest only If other functional groups of higher priority are present, or the molecule is branched to the point where naming the two carbon chains as branches is just too tedious, we can name the -O- as a part of the whole carbon chain. The prefix name for ethers is oxa. In this strategy, the longest chain with the -O- is named as the base name. The oxygen is counted as one of the carbons. A number prefix is used to designate the location of the oxygen. In this way, the base name can reflect some other functional group. Examples C-C-C-O-C-C 3-oxahexane 5 carbons + 1 ether oxygen = a 6 unit chain the oxygen is in the 3rd spot on the chain: 3-oxa C-C-C-O-C-C 5-methyl-4-oxa-2-hexanol OH is highest on the priority list: it gets the low C number | | OH C

Sample Questions

1) Give two names for the following molecules: a) C-C-O-C-C b) C-O-C-C-C c) C-C-O-C | C 2) Draw Lewis structures for the following substances. a) ethyl propyl ether a) isopropoxy butane b) dimethyl ether