Hydrocarbons

Aliphatic Series

Chains or Branched Chains

Kenneth E. Schnobrich

What Makes Carbon Special?

• Note the electron configuration of carbon in the ground state, 2-4.

• Using Lewis Dot Structures we can examine several arrangements for the electrons

C C C C

4 singlebonds

2 singlebonds &1 double bond

2 doublebonds

1 singleBond & 1 triple bond

Case 1 Case 2 Case 3 Case 4

Carbon Special cont.

• Case #1 - atoms tend to bond in a tetrahedral fashion around carbon. If they are identical atoms the molecule is nonpolar.

• Case #2 - atoms tend to bond in a trigonal planar fashion around carbon. These molecules tend to be polar.

• Case #3 - atoms tend to bond in a linear fashion around the carbon. If the bonding atoms are identical the molecule will be nonpolar.

• Case #4 - atoms tend to bond in a linear fashion around the carbon. These molecules tend to be polar.

The Alkane Series

• Members of the Alkane series all have names that end in “ane”.

• They are saturated hydrocarbons - all single bonds between carbon atoms.

• They are all members of a homologous series - they each differ by a -CH2 group.

• They all follow the general formula

CnH2n+2

• They undergo substitution reactions

MethaneCH4

EthaneC2H6

PropaneC3H8

n-ButaneC4H10

iso-Butane2 methyl ButaneC4H10

n-PentaneC5H12

iso-Pentane2 methyl ButaneC5H12

Isomers

• Notice that in Butane and Pentane the molecular formula is the same but there are different arrangements for the atoms. This defines an isomer - same formula, different arrangement.

• Look at Hexane on the next slide

Hexane & Isomers

n-HexaneC6H14

2,3 dimethyl ButaneC6H14

2,2 dimethyl ButaneC6H14

Rules for Naming

• Locate the longest continuous chain of carbon atoms (that is the base name)

• Number the carbons in the longest chain starting from that end to which the attached group (or groups) are closest

• If there is more than one of the same attached group you must use a prefix like - di, tri, tetra

• Identify the number of the carbon on the longest chain to which the groups are each attached

Let’s Use Hexane

1

2

3

4

- Notice there are four carbons in the longest continuous chain (Butane)-You can # from either end because of the location of the attached “methyl” groups- Since there are two “methyl” groups you must use the prefix “di” (dimethyl)- Notice that one is on carbon 2 and the other on carbon 3- So the final name would be2,3 dimethyl Butane

Methyl group -CH3

Methyl group -CH3

Next Isomer of Hexane

43

2

1

- Notice there are four carbons in the longest continuous chain (Butane)-You must # from the left end because of the location of the two attached “methyl” groups- Since there are two “methyl” groups you must use the prefix “di” (dimethyl)- Notice that both “methyl” groups are on carbon 2 - So the final name would be2,2 dimethyl Butane

Methyl group -CH3

Methyl group -CH3

both Methyl groups on carbon #2

Isomers

• It should be noted that as the number of carbons increases the number of possible isomers increases considerably.

• For example, Octane (C8H18), there are 18 possible isomers including a continuous chain of 8 carbons.

The Alkene Series

• Members of the Alkene series all have names that end in “ene”.

• The are unsaturated hydrocarbons - there is one double bond between two carbon atoms.

• They are all members of a homologous series - they differ by a -CH2 group

• They all follow the general formula CnH2n

• They undergo addition reactions

Alkene Series cont.

• Since the double bond can appear anywhere in the longest chain you must identify its location by numbering the bond spaces.

• Always start from that end of the carbon chain to which the double bond is closest.

Note the double bond - since there are only twobond spaces the name is simply Propene

1

2

Note the double bond - since there are only twobond spaces the name is simply Propene

2

1

123

The double bond appears in bond position #1so the compound is named Butene-1

12

3

The double bond appears in bond position #2so the compound is named Butene-2 - it doesn’tmatter which end you number from, the double bond appears in the second bond space

Note - as the number of carbons in the chain increases the number ofpossible double bond locations increases

Alkyne Series• The alkynes are hydrocarbons whose names

all end with “yne”.• The alkynes are characterized by a triple bond

between two carbon atoms somewhere in the structure.

• The alkynes are unsaturated and follow the general formula CnH2n-2.

• Alkynes undergo addition reactions at the triple bond.

Alkyne Series (cont.)

• The first member of the alkyne series is ethyne (commonly called acetylene) and its formula is C2H2.

• As the carbon chain grows longer (4 or more carbons) it is necessary to locate the triple bond in the name of the compound.

Triple bond

Alkyne Series (cont.)

• Now you must number the bond spaces (between the carbon atoms) starting from that end of the chain to which the triple bond is closest.

Triple bond Triple bondYou can tell the triple bond by the

Shorter bond distance

Butyne-1 or 1-Butyne Pentyne-2 or 2-Pentyne

12

31

23

4

General Comments

• Whether it is the alkane, alkene, or alkyne series they are considered homologous series because each member differs by the same –CH2 grouping (or structure).

• In all of the series, the greater the complexity of the structure the higher the boiling point.

General Comments (cont.)

• Alkanes (saturated hydrocarbons) undergo substitution reactions.

• Alkenes and Alkynes (unsaturated hydrocarbons) undergo addition reactions.

• Organic compounds generally have low melting and boiling points (weak intermolecular attractions).

Aromatic Hydrocarbons

• Aromatic hydrocarbons have typical, sometimes unpleasant, odors.

• Benzene, the first member of the Benzene series, has a unique ring structure with alternating double bonds.

Double bond

Double bond

Double bondJust another way of representing Benzene

Aromtic Hydrocarbons

• The second member of the Benzene series is Toluene (foul smelling).

• The general formula for the series is CnH2n-6.

-CH3