(1.2) Cyclic and Aromatic Hydrocarbons - Classification & Naming

Cyclic Hydrocarbons Hydrocarbon ring structures are called cyclic hydrocarbons. 2 ends of a hydrocarbon chain join, thus we need to remove 2 H atoms from each end. Ex. Steroids, cholesterol steroid

Cyclic Hydrocarbons: Alkanes consider hexane vs. cyclohexane cyclohexane has 2 less H atoms general formula for cylcoalkanes is: CnH2n Note: This is the same general formula for an alkene

Drawing & Naming Cyclic Hydrocarbons: Alkanes To draw: draw in a ring like shape Ex: cyclohexane To name: use the same rules as for alkanes use prefix “cyclo” before the parent chain to indicate the ring structure Ex: cyclopentane

Drawing & Naming Cyclic Alkanes 1) Branched cyclic alkanes If there is a branch, name it first. Ex. methylcyclohexane

Drawing & Naming Cyclic Alkanes 2) Two or more branches on a ring structure Numbers used to indicate the position of the branches must be the lowest possible. Any C in the ring can be #1 and we want lowest possible #, therefore if we have one branch we call that C #1 Branches are named in alphabetical order. Ex. 1-ethyl-3-methylcyclohexane Note: We can count the C’s in either direction of the ring.

Drawing & Naming Cyclic Alkenes For the double bond Its position number is given priority for lowest number Branches are numbered in order to give the lowest position numbers. e.x. 3-methylcyclohexene

Drawing & Naming Cyclic Alkynes Name and draw the same as cycloalkenes, but use“-yne” ending

Aromatic Compounds and Benzene Aromatic compounds contain benzene. Benzene, C6H6 , is represented as a six carbon ring with 3 double bonds. Two possible structures can be drawn to show benzene in this form.

Benzene Structure The structures for benzene can also be written as a single structure where the alternating double bonds are written as a circle within the ring. Benzene structure

Aromatic Compounds in Nature and Health Many aromatic compounds are common in nature and in medicine.

Naming Aromatic Compounds Single Branch – method 1 Aromatic compounds are named with benzene as the parent chain. One side group is named in front of the name benzene. methylbenzene (toluene) chlorobenzene

Naming Aromatic Compounds Single Branch – method 2 Alternatively, aromatic compounds may be named as a branch of a large molecule. The benzene ring is an alkyl group called phenyl . phenylmethane 2-phenylpropane

Naming Aromatic Compounds 2) Two or more branches – method 1 When two groups are attached to benzene, the ring is numbered to give the lower numbers to the side groups (counting in either direction). e.x.1,2-dimethylbenzene

Naming Aromatic Compounds 2) Two or more branches - method 2 prefixes are also used ortho (1,2-), meta (1,3-) and para (1,4-) 1,2-dimethylbenzene 1,3-dichlorobenzene 1-chloro-4-methylbenzene (ortho-dimethylbenzene) (meta-dichlorobenzene) (para-chloromethylbenzene) o-dimethylbenzene m-dichlorobenzene p-chloromethylbenzene

Some Common Names Some substituted benzene rings also use a common name. Then naming with additional side groups uses the ortho-, meta-, para- system.

Learning Check 1 Select the names for each structure: 1. chlorocyclohexane 2. chlorobenzene 3. 1-chlorobenzene 1. meta-methyltoluene 2. meta-dimethylbenzene 3. 1,3-dimethylbenzene

Solution 1 Select the names for each structure: 2. chlorobenzene 1. meta-methyltoluene 2. meta-dimethylbenzene 3. 1,3-dimethylbenzene

Learning Check 2 Write the structural formulas for each of the following: a) 1,3-dichlorobenzene b) ortho-chlorotoluene

Solution 2 Write the structural formulas for each of the following: a) 1,3-dichlorobenzene b) ortho-chlorotoluene