Organic Chemistry
CHEMISTRY 30
Intro to Hydrocarbons

Hydrocarbon: an organic compound that contains only
carbon and hydrogen.

E.g. methane

Recall from our last unit that carbon wants to make four
bonds.

We can represent hydrocarbons in different ways:
Lewis Structure
Properties of Hydrocarbons

Hydrocarbons with low masses tend to be gases or liquids that boil
at a low temperature (due to low IMFs)

Recall the general rule “like dissolves like.” Because of this,
hydrocarbons are insoluble in polar compounds. This explains why
oil and water do not mix!
Alkanes

An alkane is a hydrocarbon in which there are only single covalent bonds.

Each carbon-carbon bond is a single covalent bond, and every other
bond is a carbon-hydrogen bond.

Alkanes have the general formula CnH2n+2

They end with “ane” Ex: C3H8 = Propane
Alkanes

Alkanes are named by counting the number of carbons, and by using the
corresponding prefix for that number. Then you add “-ane” to represent
an alkane. You will need to memorize the names of the first ten alkanes.
Straight-Chain Alkanes

Ethane is the simplest straight-chain alkane, which is an alkane that
contains any number of carbon atoms, one after the other.

To draw the structural formula, draw each carbon connected by a bond,
and complete each carbon’s octet with hydrogens.
Stop and Check!

Draw complete structural formulas for the straight-chain alkanes that have:

Three carbon atoms

Four carbon atoms

Five carbon atoms

Name each of the above structures.

How many single bonds are there in a propane molecule?
Branched-Chain Alkanes

Carbons can connect in branched chains.

Substituent: an atom (or group of atoms) that can take the place of a
hydrogen atom on a parent hydrocarbon molecule.

The parent alkane is the longest continuous carbon chain. All other carbon
atoms or groups of atoms are called substituents.

The parent alkane of the above compound is hexane (six carbons).
Branched-Chain Alkanes

A hydrocarbon substituent that is derived from an alkane is called an alkyl
group. Think of an alkyl group as an alkane with one of the hydrogens
removed.

An alkyl group can be one or more carbons long.

Name these groups by removing the –ane ending from the parents
hydrocarbon name and adding –yl.

The three smallest alkyl groups are the methyl group (-CH3); the ethyl group
(-CH2CH3), and the propyl group (-CH2CH2CH3)
Branched-Chain Alkanes

When a substituent alkyl group is attached to a straight-chain hydrocarbon,
branches are formed. An alkane with one or more alkyl groups is called a
branched-chain alkane.

Each carbon in an organic molecule can be characterized as a primary,
secondary, tertiary, or quaternary carbon.

Primary: 1 carbon attached to it

Secondary: 2 carbons attached to it

Tertiary: 3 carbons attached to it

Quaternary: 4 carbons attached to it
Branched-Chain Alkanes
Naming Branched-Chain Alkanes

IUPAC (International Union of Practical and Applied Chemists) naming system (like everything
else we know how to name!)

We will use the following molecule:
Naming Branched-Chain Alkanes


Find the longest continuous chain of carbons in the molecule. This
is considered the parent hydrocarbon.

2
5
1
6
Step 2:


3
The longest chain has six carbons, therefore the parent
hydrocarbon is hexane.

4
Step 1:
Number the carbons in the main chain in sequence. Start at the end that will give the substituent groups
attached to it the smallest numbers.
Step 3:

Add numbers to the names of the substituent groups to identify their positions on the chain. These
numbers become prefixes to the name of the substituent group .

The substituents and positions are 2-methyl and 4-methyl.
Naming Branched-Chain Alkanes

Step 4:

Use prefixes to indicate the appearance of the same group more
4
than once in the structural formula. Common prefixes are di- (two),
3
tri- (three), and tetra- (four).


The two methyl groups are combined as 2,4-dimethyl.
Step 5:

List the names of alkyl substituents in alphabetical order.

For the purposes of alphabetizing, ignore the prefixes di-, tri-, and so on.

The 2,4-dimethyl group is our only substituent group, so we name it first.
5
6
2
1
Naming Branched-Chain Alkanes

Step 6:

Combine all the parts and use proper punctuation. Write the entire name
without any spaces.

Use commas to separate numbers and use hyphens to separate numbers and
words.

Our molecule becomes 2,4-dimethylhexane.
Naming Branched-Chain Alkanes

Name the following alkanes:
4-ethyl-2,4-dimethylheptane
4,4,5-tripropyloctane
2,2,3-trimethyl-3-ethylheptane
Drawing Branched-Chain Alkanes

Draw the structural formula for the following molecules:

octane

2,2,4-trimethylpentane

3-methylhexane

3-ethyl-3,4-dimethyloctane
Unsaturated Hydrocarbons

Saturated Compound: an organic compound that only contains single bonds.

Unsaturated Compound: an organic compound that contains double or triple
carbon-carbon bonds.

An alkene is a hydrocarbon that contains one or more carbon-carbon double
bonds.
Alkenes

Ethene is the simplest alkene. It is often called by the common name ethylene.

To name an alkene by the IUPAC system, find the longest
chain in the molecule that contains the double bond. This
chain is the parent alkene.

The parent alkene is named just like the alkane with the same number of carbons,
plus the ending –ene. The chain is numbered so the carbon atoms with the double
bond have the lowest possible numbers.

Name the substituents the same way you would for an alkane.
Alkenes

E.g. name the following alkenes:
propene
1-butene
2-butene
4-methyl-2-pentene
Alkynes

A hydrocarbon that contains one or more carbon-carbon triple bonds is
called an alkyne. Like alkenes, alkynes are unsaturated hydrocarbons.

The simplest alkyne is ethyne (C2H2), which has the common name acetylene.

They are named the same way as alkenes and alkanes.
1-propyne
4-methyl-2-pentyne
Isomers

Structures of some hydrocarbons differ only by positions of substituents or of multiple bonds.
Isobutane is also called
2-methylpropane

Compounds that have the same molecular formula but different molecular structures are called
isomers.
Constitutional Isomers

Butane and 2-methylpropane are specifically called constitutional isomers because they
have the same molecular formula but are joined together differently.

What are some other constitutional isomers?

Constitutional isomers differ in physical properties such as boiling point and melting point.
They also have different chemical reactivities.

In general, the more highly branched the hydrocarbon is, the lower the boiling point of the
isomer will be compared with less branched isomers.

n-butane boiling point: -1.0 degrees Celcius; isobutane’s boiling point: -11.7 degrees Celcius
Isomers as Substituent Groups

Alkyl groups can be organized in branched chains just like the parent hydrocarbon can.

A propyl group can either be n-propyl (n for normal) or iso-propyl (iso for isomer):
n-propyl group
iso-propyl group
Isomers as Substituent Groups

Butyl groups work the same way, as shown:

sec- and tert- represent the degree of the first carbon in the group (the one bonded to
the parent hydrocarbon)
Stereoisomers

Remember that molecules are 3-D structures. This means that molecules with the same
molecular formula and with atoms joined in exactly the same order may still be isomers.

Stereoisomers are molecules in which the atoms are joined in the same order, but the positions
of the atoms in space are different.

The two types of stereoisomers are cis-trans isomers and enantiomers. We will be focusing our
study on cis-trans isomers rather than enantiomers.
Cis-Trans Isomers

A double bond between two carbons prevents other atoms in the molecule from rotating, or
spinning, with respect to one another. Because of this lack of rotation, groups on either side of
the double bond will be ‘stuck’ in specific orientations.

Cis-trans isomers, aka geometric isomers, have atoms joined in the same order but with a
different spatial orientation.

Cis-trans isomerism occurs most frequently in molecules with double bonds.
Cis-Trans Isomers

Look at the models of 2-butene:

In the cis configuration, similar groups are on
the same side of the double bond.

In the trans configuration, similar groups are
on the opposite sides of the double bond.

Cis-trans isomers have different chemical and
physical properties.
Hydrocarbon Rings

Not all hydrocarbons are straight chains or branched chains. In some hydrocarbon
compounds, the carbon chain is in the form of a ring.

A compound that contains a hydrocarbon ring is called a cyclic hydrocarbon. Many
molecules found in nature contain cyclic hydrocarbons. Rings with 5 and 6 carbons are the
most abundant.

Cyclic hydrocarbons can be either saturated or unsaturated. A cyclic hydrocarbon that
contains only single bonds (and therefore is saturated), is called a cycloalkane.
Cyclic Hydrocarbons

To name a cycloalkane, count the number of carbons in the ring and assign the
corresponding alkane name. Then simply add the prefix cyclo- to the alkane name.
Cyclic Hydrocarbons

Substituents are named just as they would be for an alkane.
1-ethyl-3-methylcyclohexane
1-methyl-2-propylcyclopentane
Aromatic Hydrocarbons

There is a class of unsaturated cyclic hydrocarbons that are responsible for the aromas of
spices such as vanilla, cinnamon, cloves, and ginger. These were classified as aromatic
compounds because of their pleasant aromas. However, not all aromatic compounds
even have a smell at all.

An aromatic compound is defined as an organic compound that contains a benzene
ring, or another ring in which the bonding is like that of benzene.

Any compound not classified as aromatic is called an aliphatic compound. All of the nonaromatic compounds we’ve studied so far are aliphatic.
Aromatic Hydrocarbons

Benzene: a six-carbon ring with one hydrogen attached at each carbon.

This arrangement leaves one electron from each carbon free to participate in a double
bond. Benzene is drawn as such:
Aromatic Hydrocarbons

Benzene can be drawn using resonance structures:

The bonding electrons between carbon atoms are
shared evenly around the ring.

Benzene is often drawn with a circle inside the
hexagonal structure to demonstrate that those
electrons are shared evenly.

Benzene and other molecule that exhibit resonance
are more stable than similar molecules that do not
exhibit resonance, making it less reactive.
Naming Aromatic Compounds

Compounds containing substituent groups
 When the benzene group is a substituent, the
attached to a benzene ring are named using
C6H5 group is called a phenyl group.
benzene as the parent hydrocarbon.
methylbenzene
ethylbenzene
3-phenylhexane