The Chemistry of Carbon A very brief introduction Organic The Chemistry of Carbon A very brief introduction

Alkanes – saturated carbon chains The simplest organic molecules contain only carbon and hydrogen, which are connected to each other by only single covalent bonds. They are referred to as saturated, because no more hydrogen can be bonded to any carbon atoms. The alkanes that exist in strings or chains are called: normal, straight-chain, or unbranched hydrocarbons.

All normal alkanes have the generic formula CnH2n+2 and the chains will zig-zag with a bond angle between each carbon atom being 109.5°. They are a string of tetrahedral molecules joined together. Names of the alkanes derive from the number of carbons present in the chain. Meth- 1 carbon hex- 6 carbons Eth- 2 carbons hept- 7 carbons Prop- 3 carbons oct- 8 carbons But- 4 carbons non- 9 carbons Pent- 5 carbons dec- 10 carbons The ending –ane is added to the appropriate prefix to name unbranched alkanes.

Representing structures of alkanes There are three methods commonly used to represent organic structures; most commonly the dash formula and condensed formula, and the bond line formula.

Bond line formula

Structural Isomers Butane, 4 carbons, and above have more than one possible arrangement of atoms that can represent the same formula.

Molecules that have the same formula but different arrangement of the atoms are called isomers. Sometimes things appear to be isomers but are actually the same structure.

Draw 5 structural isomers of hexane, C6H14 2-methylpentane 3-methylpentane 2,2-dimethylbutane 2,3-dimethylbutane

Naming alkanes The names of unbranched alkanes we already discussed, but what about alkanes with branches? When we have branched alkanes, there are many groups that can form the branch. The most common of these groups are called alkyl groups, and consist of an alkane that has lost one hydrogen atom, thus allowing the group to attach to a chain of carbons.

The name of the compound will be taken from the longest continuous carbon chain in the branched molecule. The longest chain is 5 carbons, a pentane. Off one of these carbons is a methyl group. To properly name the compound, we must identify where the branch occurs and what branches off. The chain is numbered so that the branch is at the lowest numbered carbon.

2-ethylpentane,3-methylhexane or 4-methylhexane? 3-methylhexane. Longest chain with substituent group off lowest numbered carbon.

3-methylpentane 2,3-dimethylbutane

4-ethyl-3,5-dimethylnonane

4-tert-butylheptane

Cyclic alkanes Sometimes the alkanes exist in a chain that connects to itself, having the general formula CnH2n. These cyclic alkanes will be named according to the number of carbons in the ring.

Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cyclo hexane is unique in that it exists in two forms or conformations, chair and boat.

Ethylcyclobutane 1-methyl-3-propylcyclopentane 2-ethyl-1,3-dimethylcyclohexane

Multiple bonds- alkenes and alkynes When multiple bonds exist in a hydrocarbon chain, the name must incorporate where the multiple bond sits as well as all the substituent groups. The numbering of the carbon chain is such that the multiple bond get the lowest possible number.

Simplest alkene Ethene- The double bond restricts rotation and the molecule is flat, planar, in geometry.

2-butene 2- pentyne 2,3-dimethyl-1-pentene

Functional groups- other branches Most organic compounds are derivatives of hydrocarbons. They contain elements other than just carbon and hydrogen. These other elements usually exist in about 8 different forms or functional groups. Each of these groups will have its own naming rules. We will learn basic naming and recognition of the functional groups.