Organic Chemistry: Hydrocarbons Essential Concepts
Organic chemistry deals with compounds that contain carbon. About 10 million different organic compounds are known -- far more than all known inorganic compounds. Carbon is unique among elements in that it is capable of bonding in several different ways, with atoms of many other elements, and with other carbon atoms to form long chains. You don't need to study each of the millions of organic compounds to understand organic chemistry because they can be classified into groups of compounds that have similar structures and properties.
Objective 1 Write and interpret structural formulas of linear, branched, and cyclic alkanes, alkenes, and alkynes.
Organic compounds contain backbones of carbons bonded to one another. Carbon, as you may remember, has four valence electrons, which it is eager to share with other atoms to obtain a stable octet of electrons. Carbon thus normally forms four covalent bonds with other atoms. Possible combinations of bonds are illustrated below (notice, in particular, the center carbon).
4 single bonds C – C – C
2 single bonds,C – C = C
1 double bond
2 double bonds C = C = C
1 single bond, HC – C C
1 triple bond
To make everything easier, the carbon – hydrogen bonds are usually not included in structural formulas. The four structures illustrated above are more conveniently written as:
CH3 – CH2 – CH3, CH3 – CH = CH2, CH2 = C = CH2, and CH3 – C CH
Alkanes (also known as saturated hydrocarbons) are made of only carbon and hydrogen with the carbon atoms connected to each other by singlebonds (no double or triple bonds). (Because alkanes have only single bonds, they are bonded to a maximum number hydrogens. They can’t accept more and are thus saturated.)
Alkenes contain at least one double bond between carbon atoms. They, like alkynes described below, are unsaturated.
Alkynes contain at least one triple bond between carbon atoms.
Organic compounds can be:
1) linear CH3 – CH2 – CH2 – CH2 – CH2 – CH3
2)branched CH3 – CH2 – CH – CH2 – CH3
CH3
3)ringed (cyclic) CH2 – CH2
CH2 CH2
CH2 – CH2
Obj 2 Name unbranched organic compounds
Names of unbranched organic compounds (commonly used names are in parentheses)
Alkanes / Alkenes (note: indicate which carbons are double bonded. Eg: 1-butene) / Alkynes (note: indicate which carbons are triple bonded. Eg: 2-butyne)1 carbon / methane / -- / --
2 carbons / ethane / ethene (ethylene) / ethyne (acetylene)
3 carbons / propane / propene / propyne (propylene)
4 carbons / butane / butene / butyne
5 carbons / pentane / pentene / pentyne
6 carbons / hexane / hexene / hexyne
7 carbons / heptane / heptene / heptyne
8 carbons / octane / octene / octyne
9 carbons / nonane / nonene / nonyne
10 carbons / decane / decene / decyne
Obj 3 Name branched organic compounds
Steps:
1)Find the longest continuous chain of carbon, consider it the parent chain.
2)Determine how many branches are on the longest chain and their sizes.
3)Starting from the end of the parent chain that is closest to a branch, indicate the carbon position numbers from which the branches arise.
4)Change the suffixes of the branches from –ane to –yl. (a 1 carbon branch is methyl)
5)If the parent chain has two identical branches, use the prefix di- in front of the branch name. If the branches are not identical, list them alphabetically.
6)Put the name together. List the position numbers of the branches followed by a hyphen, and then the names of the branches (listed alphabetically), and end with the name of the parent chain.
4-ethyl-3methylheptane has a seven carbon parent chain, an ethyl group (2 carbons) attached to the 4th carbon of the parent chain and a methyl group (1 carbon) attached to the 3rd carbon of the parent chain. Its structural formula would be:
CH3 CH2CH3
CH3 – CH2 – CH – CH – CH2 – CH2 – CH3
Obj 4 Name alkanes containing rings.
To name ringed alkanes, use the same rules but the prefix cyclo- is placed before the name. Thus a ring with six carbons would by cyclohexane.
Obj 5 Describe the properties and uses of alkanes, alkenes, and alkynes.
Alkanes
In general, as the number of carbons in alkane chains increases, the melting and boiling points increase. Thus small alkanes (methane, ethane, propane, and butane) are gases at room temperature, alkanes with 5 to 16 carbons are liquids, and those with more than 16 carbons are solid.
Alkanes are relatively unreactive and are non-polar (and will dissolve other non-polar substances). Thus they make good organic solvents are used in paints, paint removers, and cleaning solutions.
Alkenes and alkynes
The melting and boiling points of alkenes and alkynes are lower than alkanes. This means that while an 18 carbon alkane would be a solid, an 18 carbon alkene would be liquid.
Alkenes are more reactive than alkanes and readily undergo synthesis reactions in which smaller molecules or ions bond to the atoms on either side of the double bond. When corn oil, an unsaturated fat (containing alkenes) is “hydrogenated” (the double bonds are broken and hydrogens are accepted to saturate the compound) solid margarine, a saturated fat, is produced (containing alkanes).
Alkynes are very reactive and, because of this, are rarely found naturally but can be produced from other organic compounds.
Obj 6 Describe sources of organic compounds
Most hydrocarbons come from fossil fuels, especially petroleum, but also natural gas and coal. Other important sources include wood and fermentation products of plant materials. Petroleum is a complex mixture of hydrocarbons, which must be separated before they can be used. Fractional distillation is one method used to separate fractions.