Introduction to Organic Chemistry

Defining “Organic”

• An organic compound is one containing carbon.• Exceptions:

• Carbon oxides

• Carbides

• Carbonates

Millions of Compounds• Carbon is unique among elements in that it

can bond to other carbon atoms to form chains containing as many as several thousand atoms.

C

The Lewis structure for carbon shows that it has four valence electrons, so can form four bonds.

• Because a carbon atom can bond to as many as four other atoms at once, these chains can have branches and form closed-ring structures that make possible an almost endless variety of compounds.

• In addition, carbon can bond strongly to elements such as oxygen and nitrogen, and it can form double and triple bonds.

• Thus, carbon forms an enormous number of compounds with chains and rings of various sizes, each with a variety of bond types and atoms of other elements bonded to them.

• Fortunately, you don’t need to study each of these millions of compounds to understand organic chemistry because they can be classified into groups of compounds that have similar structures and properties.

The simplest organic compound• The simplest organic compounds are

hydrocarbons.• Hydrocarbons contain only two elements:

• carbon• hydrogen

• Two main types of hydrocarbons:• saturated• unsaturated

Saturated Hydrocarbons

• A hydrocarbon in which all the carbon atoms are connected to each other by single bonds is called a saturated hydrocarbon.

• Another name for a saturated hydrocarbon is an alkane.

• Alkanes are the simplest hydrocarbons.

Properties of Alkanes

• Properties depend on the structure or arrangement of atoms present in a molecule.

• Another factor that affects properties of alkanes is chain length.• In general, the more carbons present in a straight-

chain alkane, the higher its melting and boiling points.

• A property shared by all alkanes is that they are relatively unreactive.

• The carbons in an alkane can be arranged in a chain or a ring, and both chains and rings can have branches of other carbon chains attached to them.

CH3 CH

CH3

CH2 CH CH2CH2CH3

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2 CH2

CH2

CH

C

CH2

CH3CH3

CH2

CH3

These are the names of the first ten alkanes and their molecular formulas.KNOW them

• Alkanes that have no branches are called straight-chain alkanes.

• Most alkanes have a branched structure.

• In these compounds, a chain of one or more carbons is attached to a carbon in the longest continuous chain, which is called the parent chain.

CH3 CH CH3

CH3

• The parent chain is 3 carbons long. It is propane. There is a 1-carbon branch off the second carbon (carbon #2).

CH3 CH CH3

CH31-carbon branch off the second carbon

parent chain has 3 carbons, so it is propane

To name a branched alkane, you must be able to answer three questions about its structure.

1. How many carbons are in the longest chain? (parent chain)

2. How many branches are on the longest chain and what are their sizes?

3. To which carbons in the parent chain are the branches attached?

• For convenience, the carbon atoms in organic compounds are given position numbers.

In straight-chain hydrocarbons, the numbering can begin at either end. It makes no difference.

CH3 CH2 CH2 CH31 2 3 4

1234

• In branched hydrocarbons, the numbering begins at the end closest to the branch.

CH3 CH CH2 CH3

CH3

1 2 3 4

Four carbons are in the longest continuous chain, so butane is the parent chain and will be part of the compound’s name.

CH3 CH CH2 CH3

CH3

•There is only one branch, and it contains one carbon.

CH3 CH CH2 CH3

CH3

Instead of calling this a methane branch, change the -ane in methane to -yl. Thus, this is a methyl branch.

Because the methyl branch is attached to the second carbon of the butane chain, this compound has the name 2-methylbutane.

CH3 C CH3

CH3

CH3

Now, examine the structure of a different hydrocarbon.

12

3

Propane will be part of this compound’s name because the longest continuous chain has three carbons.

Two methyl branches are present, both on the second carbon.

CH3 C CH3

CH3

CH3

To indicate the presence of more than one branch of the same kind, use the same Greek prefixes for naming covalent compounds.

The prefix to use when two of anything are present is di-. Thus, the name of this compound is 2,2-dimethylpropane.

Isomers

• Compounds that have the same molecular formula but different structures are called isomers.

• Butane and 2-methylpropane are known as structural isomers.

• Each has the molecular formula C4H10, but they have different structural formulas because the carbon chains have different shapes.

CH3 CH2 CH2 CH3

CH3 CH CH3

CH3

Butane, C4H10

2-methylpropane, C4H10

Halocarbons• Sometimes, a halogen can be substituted for a

branch in a hydrocarbon.

• These compounds are called halocarbons.

• Halocarbons are named using the beginning of the name of each halogen:

• Fluoro-

• Chloro-

• Bromo-

• Iodo-

CH3 CH CH3

Cl

This is 2-chloropropane

Cycloalkanes

• Alkanes can also be arranged into carbon rings.

• These are called cycloalkanes.

• They are named just like chain alkanes with the prefix cyclo-.

Cycloalkanes

• Here is the simplest cycloalkane:

cyclopropane

CH2

CH2 CH2

Cycloalkanes

cyclopropane

cyclobutane

cyclopentane

These can be drawn as simple geometric shapes, the corners represent carbons and the hydrogen atoms are left off but understood to be there.

Name this alkane!

CH3 CH CH2

CH3

CH32-methylbutane

Name this alkane!

2,3-dimethylpentane

CH2 CH CH

CH3

CH3

CH3

CH3

Name this alkane!

3-ethyl-4-methytlheptane

CH2 CH CH

CH3

CH2

CH2

CH2

CH3

CH3CH3

Name this alkane!

cyclohexane

Name this alkane!

3-ethyl-2,3-dimethylhexane

C C

CH3

CH3

CH3

CH2

CH2H CH2

CH3

CH3

Name this alkane!

CH3

CH3

1,2-dimethylcyclohexane