Aromatics & Cyclic Aliphatics

1. Aromatics & Cyclic Aliphatics

2. Aromatics and Aliphatics Both types of compounds: • Are hydrocarbons (contain H & C) • Maintain stable carbon octets (8 valence electrons) • Are flammable • Have low boiling points

3. Aromatics and Aliphatics 1. Aliphatic compounds • Any organic compound that isn’t aromatic • May be straight chained, branched-chained or cyclic • May have single, double or triple bonds • Most are flammable 2. Aromatic compounds • Rings that are unexpectedly stable • Resonance- delocalization of electrons  bonds that are more stable than you would expect due to the ability of the electrons to alternatively single and double bond • This is often depicted using Kerukle or resonance structures

4. Cyclic Hydrocarbons • Numbering may go in a clockwise or counter-clockwise fashion • Number so that an alkyl group is at the 1 position and the others are numbered as low as possible • Cyclic nature is indicated by the parent chain prefix “cyclo” • If you have one methyl group it becomes part of the parent chain • Remember: • Name alphabetically • Use di, tri, etc. to indicate multiples • Each corner is a carbon...

5. Examples of Cyclic Hydrocarbons

6. Aromatic Hydrocarbons • The name “aromatic” originates from the fact that all aromatic compounds were associated with distinct, pleasant odours • Vanilla, ginger, cinnamon • This is not true currently, as some aromatics are odourless • Aromatics may contain one or more rings • With the defining feature being that one ring is either benzene or has bonding properties similar to benzene

7. Aromatic Hydrocarbons • Benzene • A six-membered carbon ring with a hydrogen atom attached to each carbon • This leaves one electron from each carbon available to form a double bond • There are two different structures for benzene: • These are called resonance structures

8. Benzene • However these two structures only illustrate the extremes of electron-sharing • In actuality all the C-C bonds in benzene are identical hybrids of single and double bonds • For this reason, benzene may also be represented as: • However this does not illustrate the number of electrons involved, therefore the traditional structure is preferred

9. Substituted Aromatic Compounds • Dyes responsible for producing bright colours contain substituted aromatic compounds • For naming there are two options: • When there are substituents attached to a benzene ring it is named alkyl group benzene • Example: ethylbenzene • When a benzene ring is a substituent on an alkane it is called a phenyl group • Example: 3-phenylhexane

10. Disubstituted Benzenes • When a benzene ring has two substituents • Most of these compounds will have three isomers: • Example: dimethylbenzene = xylene • Isomers occur at three known positions • Ortho (1,2)  ortho-xylene • Meta (1,3)  meta-xylene • Para (1,4)  para-xylene

11. Disubstituted Benzenes • Some benzenes will have two halogens attached • Naming is similar to that of alkyl groups but the halogen root is used instead • Don’t forget to: • Indicate position using numbers • Indicate multiples using di, tri, etc. • Number groups as low as possible • Name compounds alphabetically

12. Disubstituted Benzenes • Example: • 1,4-dichlorobenzene ORpara-chlorobenzene • 1-bromo-2-chlorobenzene

13. “Special” Benzenes Toluene – a methyl group on a benzene Aniline – an NH2 group on a benzene Napthalene – two conjoined benzenes Anthracene – three conjoined benzenes Phenol – a OH group on a benzene Xylene – two methyl groups on a benzene