Carbon Chemistry

Classification of Hydrocarbons and Organic Functional Groups

The Families of Hydrocarbons:alkanes, alkenes, alkynes, and aromatics

Alkanes, revisited• Alkanes are saturated hydrocarbons with linear, branched,

and even cyclic structures.• Linear or branched molecules with the same molecular

formula are called “isomers” of each other, e.g. CH3CH(CH3)CH3 is an isomer of CH3CH2CH2CH3.

• Names of alkanes are derived from the number of carbons in the molecule (but naming is complicated with branching).

• Physical properties of linear alkanes vary systematically, e.g. Boiling point increases steadily with increasing Mr (branching tends to alter the trend, however).

• Chemically, alkanes are not very reactive, but what is important is that all hydrocarbons can undergo combustion!

Alkenes• Like alkanes, alkenes can be linear or branched.• Alkenes contain a carbon-carbon double bond

and so have fewer hydrogens per molecule than alkanes.

• The simplest alkene is called “ethene” and has the structural formula, CH2CH2.

• The position of the double bond along the carbon chain determines a different alkene. For example, CH2CHCH2CH3 (but-1-ene) is a different alkene from CH3CHCHCH3 (but-2-ene).

• Alkenes are much more chemically reactive than alkanes due to the carbon-carbon double bond.

Examples of Alkenes

C C C C

H

H H H

H

H

H

H

CH2CHCH2CH3 , but-1-ene

C C C C

H

H

H H H H

H

H

CH3CHCHCH3 , but-2-ene

Other Hydrocarbons**

• Hydrocarbons that contain a carbon-carbon triple bond are called “alkynes.” An example is ethyne (acetylene), a welding torch fuel.

• Hydrocarbons that form rings of alternating double bonds between carbons are called “aromatics.” An example is benzene, a powerful solvent.

** We will not study these other families of hydrocarbons.

Concept of Concept of “Saturation”“Saturation”

• Alkanes are “saturated” with hydrogen; they contain the maximum number of hydrogen atoms per carbon atom possible.

• Alkenes, for example, are “unsaturated.” They have fewer hydrogen atoms per carbon atom than alkanes.

• Unsaturated compounds have at least one double or triple bond between carbon atoms.

• E.g., “unsaturated fats” in foods contain one or more C=C bonds in their carbon chain

Structure of a Natural Fat

Effect of Double Bonds on Fat Structure

Beef Fat Flax Seed Oil

C=C bonds cause stiff kinks along the chains, weaken the attraction between chains, and lower the melting point of fats. At room temperature, saturated fats are solids, but unsaturated fats are liquids.

Exercise 1

1. Name the molecule with structural formula, CH3CHCH2.

2. Draw the complete structural diagrams for all the unique isomers of the alkene, C5H10. Use the molecular models to help sort out the differences.

3. Complete this table: Fill in “liquid” or “solid”

Type of Fat Room Temperature In refrigerator

Saturated

Mono-unsaturated

Polyunsaturated

Organic Functional Groups• “Functional groups” can introduce elements other

than carbon and hydrogen.• We will study only one important functional group

which is formed by incorporating the element oxygen in the molecule as –O-H (“hydroxyl group”).

• Functional groups are so-named because they result in an organic compound with a whole new function, i.e., they significantly alter the physical and chemical properties of the parent hydrocarbon.

• Functional groups (along with isomers) are mainly responsible for the extensive variety of known natural and man-made molecules.

Exercise 21. Alkanes (linear or branched) have the general molecular

formula CnH2n+2. What is the general formula for alkenes?

2. Simple alcohols are like alkanes in which one of the –H atoms is replaced by an –O-H “hydroxyl group”. Write the molecular formula and draw the structural diagram of the SIMPLEST possible alcohol. Alcohols are named like alkanes, but have “ol” at the end instead of “ane.” Name this simplest alcohol.

3. Draw a structural diagram for each of the possible linear and branched isomers of the alcohols derived from the alkane, butane (hint: an alcohol will have only one hydroxyl group per molecule).

AlcoholsCH3 OHThe functional group of alcohols

(called the “hydroxyl” group)

Example alcohol: ethanol (see below *)

Molecular Structural Structuralformula formula diagram

C2H6O CH3CH2OH C C O

H

H

H H

H

H

*Derived from the hydrocarbon, ethane. Compare the molecular formula, structural formula, and structural diagram of ethane.

Comparing Ethanol with Ethane Ethanol Ethane

Physical state at 20oC liquid gasBoiling Point 78oC -89oCColor colorless colorlessOdor “alcohol” noneCombustible (a fuel) yes yesEnergy of combustion 30 kJ/g 48 kJ/gSolubility in water miscible insolublePrimary source synthetic naturalBiochemical yes no

Some Uses for DifferentTypes of Organic Compounds

• Alkanes – fuels, solvents, lubricants

• Alkenes – solvents, building blocks*

• Alcohols – fuels, solvents, beverages, antiseptics, building blocks*

* for other organic molecules

ibuprofen(Tylenol)

glutamic acid(a protein building block)

glucose (a sugar)[O in red and H in white]

More complex molecules