Organic ChemistryCHEMISTRY 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

Step 1:

Find the longest continuous chain of carbons in the molecule. This

is considered the parent hydrocarbon.

The longest chain has six carbons, therefore the parent

hydrocarbon is hexane.

Step 2:

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.

13 2

4

5

6

Naming Branched-Chain Alkanes

Step 4:

Use prefixes to indicate the appearance of the same group more

than once in the structural formula. Common prefixes are di- (two),

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.

13 2

4

5

6

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.

Compounds that have the same molecular formula but different molecular structures are

called isomers.

Isobutane is also called 2-methylpropane

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

non-aromatic 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 attached to a benzene ring are named using benzene as the parent hydrocarbon.

When the benzene group is a substituent, the C6H5 group is called a phenyl group.

methylbenzene ethylbenzene3-phenylhexane