Compounds of Carbon

Compounds of Carbon

Carbon is an important element as it forms 90% of all chemical compounds and forms the basis of living systems. The chemistry involving carbon was known as organic chemistry, as this term indicated carbons’ importance in living organisms. Carbon is most commonly found in compounds containing hydrogen, oxygen, nitrogen, sulfur and chlorine. Proteins, carbohydrates and fats, used by our body contain C, H and O. Proteins can also contain N, S and others.

Why does Carbon form so many compounds?

As discussed earlier, carbon is an interesting element as it has an electronic configuration of 1s22s22p2. This means it has 4 valence electrons in its outer shell, which is a half full outer shell. To obtain a full outer shell, and being a non-metal, it forms covalent bonds by sharing all of its valence electrons. It can form single or double bonds with other elements.

Hydrocarbons

Hydrocarbons are compounds containing hydrogen and carbon. We commonly find hydrocarbons in the form of fossil fuels: natural gas, coal and oil. They can be classified into several families, sharing common structures and properties, as they increase in size. These are known as homologous series’: homo means ‘same’ and genos means ‘kind’.

Alkanes: The alkane homologous series have a general formula CnH2n+2 “Where n is an integer” (remember to always specify this!) and begins with methane: CH4 Each next alkane in the series differs by CH2. Crude oil consists largely of alkanes with n = 5-70.

In alkanes:

·  Each C has a single covalent bond to 4 other atoms

·  Each H has single covalent bonds to one C

·  The atoms are arranged in a tetrahedral manner

It is best to use structural formulae to represent hydrocarbons. Always draw the carbons first, connecting them into a chain, adding hydrogen and other elements later.

FORMULA / NAME
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane

Hydrocarbons can form straight chain molecules which are linear overall, even though each carbon still forms a tetrahedron in three dimensional space. Branched chain molecules have an overall shape which is not linear. The fact that the carbons can join together in different shapes results in structural isomers: molecules with the same molecular formula but different arrangements of their atoms. Structural isomers have similar chemical properties but different physical properties.

Try drawing two structural isomers for Butane C4H10 (one straight chain, one branched)

C20H42 has 366319 possible isomers!

Alkanes are considered saturated hydrocarbons as each C atom has formed its maximum number of bonds. There are no other bonds available. So, each carbon only has single bonds to others and they are not very reactive compounds.

Alkenes: Alkenes have the general formula CnH2n. They begin with C2H4, ethene, and also form a homologous series with each compound differing by CH2. Alkenes can form isomers and various isomers can even be present in one sample of a compound with a single molecular formula. Alkenes are considered unsaturated hydrocarbons: They contain at least one carbon to carbon double bond. They have less than the maximum possible bonds with hydrogen.

FORMULA / NAME
Ethene
propene
butene

Ø  Draw the isomers of C4H8

Naming Carbon compounds

Naming hydrocarbons is easy if you learn and follow some simple rules:

·  Learn table 8.4 for the prefixes of all hydrocarbons

(1) For straight chain hydrocarbons, the name of the hydrocarbon ends in:

·  An alkane (only carbon to carbon single bonds, saturated): ane

·  An alkene (a carbon to carbon double bond, unsaturated): ene

·  An Alkyne (a carbon to carbon triple bond, unsaturated): yne

(2) For unsaturated hydrocarbons:

·  Number all the carbons in the chain. Number the carbons in the direction which means that the first carbon involved in the double/triple bond will have the lowest possible number. Place this number in front of the name.

(3) For branched hydrocarbons:

·  Identify the longest continuous chain of carbon

·  Identify the name of the hydrocarbon formed by the longest continuous chain of carbon.

·  Are any single/double/triple bonds present? If so, follow steps outlined at (2)

·  Identify the alkyl group.

o  This is the left over group which replaces a hydrogen on one of the central carbons. Alkyl groups are named after the alkane from which it formed, minus one H atom which is where it joins to the longest carbon chain. CH3 is a methyl group, derived from methane CH4, minus a hydrogen.

o  You may need to indicate the position that this group is attached to on the longest carbon chain. Do this by placing the appropriate number in front of the name.

·  Overall, place the alkyl group name first, followed by the name of the longest carbon chain.

·  When two, or more identical branches occur on different carbon atoms, the prefixes di-, tri-, and tetra- are used.

·  When you have a Cl- it becomes chloro, Br- becomes bromo, I- becomes iodo

A number of forces can act in and between hydrocarbon molecules:

·  The intra-molecular forces are covalent.

·  Hydrocarbons containing only hydrogen and carbon are considered non-polar molecules. Dispersion forces act between hydrocarbon molecules, attracting them to each other. The strength of the dispersion forces increases as the size of the molecules increase, as there are more atoms in each molecule containing charged particles to attract each other.

In smaller molecules, with less intermolecular dispersion forces, less energy is required to overcome these forces and the liquids will evaporate easily. These sorts of liquids are known as volatile. These will also flow freely due to the weak forces in the liquid and are said to have low viscosity. Substances with high viscosity have a thicker texture, like honey.

Functional Groups

A hydrogen on a hydrocarbon molecule can be replaced by other elements. Doing so can change the chemical properties of the hydrocarbon as, often, we are changing it from a non-polar molecule to a polar one. The atom or group of atoms replacing a hydrogen on a hydrocarbon, influencing the properties of compounds in an homologous series, is known as a functional group. Hydrocarbons containing a particular functional group can form their own homologous series, with predictable properties.

For example, ethane C2H6 belongs to the alkane homologous series but can be used to derive molecules belonging to the Alcohol and Carboxylic Acid Homologous series’:

Draw the structural formula of ethane from the semi-structural formula CH3CH3:
Remove a hydrogen and add an –OH to form Ethanol CH3CH2OH (the “alcoholic content” of drinks): / Remove a hydrogen and add an –COOH to form Ethanoic acid CH3COOH:
This belongs to the Alcohol Homologous series
Functional group= OH (hydroxy) / This belongs to the Carboxylic Acid Homologous series
Functional group= COOH (carboxy)

Alcohols

The molecules in the Alcohol homologous series are known as ‘alkanols’. The stem of their names is derived from the alkane which has the same number of carbons. The name ends in ‘ol’ which represents the presence of the hydroxy functional group -OH.

For example, methanol has _____ carbons and an –OH functional group. Draw its structural formula:

(Remember: Alkanes are CnH2n+2 then subtract H and add an OH)

The -OH functional group makes the molecules polar. Dispersion forces continue to act between the molecules, but Hydrogen bonding becomes more prevalent. Consequently, the attraction between molecules is stronger than in normal alkanes and this is reflected by a boiling temperature which is higher than in the corresponding alkane.

Carboxylic Acids

The molecules in the Carboxylic Acid homologous series also have the stem of their name derived from the alkane which has the same number of carbons. The name ends in ‘anoic acid’ which represents the presence of the carboxy functional group -COOH.

For example, propanoic acid has _____ carbons and an –COOH functional group. Draw its structural formula:

(Remember: Alkanes are CnH2n+2 then subtract H and add a COOH)

The -COOH functional group makes the molecules polar. Dispersion forces continue to act between molecules, but Hydrogen bonding is more prevalent. Consequently, the attraction between molecules is stronger than in normal alkanes and this is reflected by a boiling temperature which is higher than in the corresponding alkane.

Even when comparing methane with methanoic acid, it is observed that the molecular size is also significantly larger, increasing the dispersion forces operating at any one time. Draw their structural formulae:

Amines

Organic compounds that contain the –NH2 functional group are called amines. Amines are soluble in water due to the formation of hydrogen bonds. This solubility decreases as the size of the alkyl group becomes larger. The amines are named by adding amino to the name of the alkane.

Ø  Draw aminomethane and 2-aminobutame