SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Carbon and Organic Chemistry
Hydrocarbons, as the name implies, are compounds containing only carbon and hydrogen in their structures. The simplest hydrocarbon is methane, CH4. As previously mentioned, this type of formula, which shows the kinds of atoms and their respective numbers, is called an empirical formula. In organic chemistry this is not sufficient to identify the compound it is used to represent. For example, the empirical formula C2H6O could denote either an ether or an ethyl alcohol. For this reason, a structural formula is used to indicate how the atoms are arranged in the molecule. The ether of C2H6O looks like this:
whereas the ethyl alcohol is represented by this structural formula:
Organic chemistry makes use of structural formulas to show atomic arrangements.
To avoid ambiguity, structural formulas are more often used than empirical formulas in organic chemistry. The structural formula of methane is
Alkane Series (Saturated)
Methane is the first member of a hydrocarbon series called the alkanes (or paraffin series). The general formula for this series is CnH2n+2, where n is the number of carbons in the molecule. Table 14 provides some essential information about this series. Since many other organic structures use the stem of the alkane names, you should learn these names and structures well. Notice that, as the number of carbons in the chain increases, the boiling point also increases. The first four alkanes are gases at room temperature; the subsequent compounds are liquid, then become more viscous with increasing length of the chain.
Alkanes are CnH2n+2. They are homologous.
Since the chain is increased by a carbon and two hydrogens in each subsequent molecule, the alkanes are referred to as a homologous series.
The alkanes are found in petroleum and natural gas. They are usually extracted by fractional distillation, which separates the compounds by varying the temperature so that each vaporizes at its respective boiling point.
Learn the names of the first 10 alkanes.
When the alkanes are burned with sufficient air, the compounds formed are CO2 and H2O. An example is:
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)
The alkanes can be reacted with halogens so that hydrogens are replaced by a halogen atom: These are called alkyl halides.
Some common substitution compounds of methane are:
NAMING ALKANE SUBSTITUTIONS. When an alkane hydrocarbon has an end hydrogen removed, it is referred to as an alkyl substituent or group. The respective name of each is the alkane name with -ane replaced by -yl. These are called alkyl groups.
Replace -ane with -yl to form alkyl groups.
One method of naming a substitution product is to use the alkyl name for the respective chain and the halide as shown above. The halogen takes the form of fluoro-, bromo-, iodo-, and so on, depending on the halogen, and is attached to an alkane name. It precedes the alkane name, as shown above in bromomethane and 1-chlorobutane.
The IUPAC system uses the name of the longest carbon chain as the parent chain. The carbon atoms are numbered in the parent chain to indicate where branching or substitution takes place. The direction of numbering is chosen so that the lowest numbers possible are given to the side chains. The complete name of the compound is arrived at by first naming the attached group, each of these being prefixed by the number of the carbon to which it is attached, and then the parent alkane. If a particular group appears more than once, the appropriate prefix (di, tri, and so on) is used to indicate how many times the group appears. A carbon atom number must be used to indicate the position of each such group. If two or more of the same group are attached to the same carbon atom, the number of the carbon atom is repeated. If two or more different substituted groups are in a name, they are arranged alphabetically.
EXAMPLE 1 2,2-dimethylbutane
Numbers have been added to the longest chain for identification only.
EXAMPLE 2 1,1-dichloro-3-ethyl-2,4-dimethylpentane
EXAMPLE 3 2-iodo-2-methylpropane
CYCLOALKANES. Starting with propane in the alkane series, it is possible to get a ring form by attaching the two chain ends. This reduces the number of hydrogens by two.
Cycloalkanes form single-bonded ring compounds.
Cycloalkanes are named by adding the prefix cyclo- to the name of the straight-chain alkane with the same number of hydrocarbons, as shown above.
When there is only one alkyl group attached to the ring, no position number is necessary. When there is more than one alkyl group attached to the ring, the carbon atoms in the ring are numbered to give the lowest numbers possible to the alkyl groups. This means that one of the alkyl groups will always be in position 1. The general formula is CnH2n.
Here is an example:
If there are two or more alkyl groups attached to the ring, number the carbon atoms in the ring. Assign position number one to the alkyl group that comes first in alphabetical order, then number in the direction that gives the rest of the alkyl groups the lowest numbers possible.
Because all the members of the alkane series have single covalent bonds, this series and all such structures are said to be saturated.
If the hydrocarbon molecule contains double or triple covalent bonds, it is referred to as unsaturated.
PROPERTIES AND USES OF ALKANES. Properties for some straight-chain alkanes are indicated in Table 14. The trends in these properties can be explained by examining the structures of alkanes. The carbon-hydrogen bonds are nonpolar. The only forces of attraction between nonpolar molecules are weak intermolecular, or London dispersion, forces. These forces increase as the mass of a molecule increases.
The table also shows the physical states of alkanes. Smaller alkanes exist as gases at room temperature, while larger ones exist as liquids. Gasoline and kerosene consist mostly of liquid alkanes. Seventeen carbons are needed in the chain for the solid form to occur. Paraffin wax contains solid alkanes.
The differences in the boiling points of mixtures of the liquid alkanes found in petroleum make it possible to separate the various components by fractional distillation. This is the major industrial process used in refining petroleum into gasoline, kerosene, lubricating oils, and several other minor components.
Alkene Series (Unsaturated)
The alkene series has a double covalent bond between two adjacent carbon atoms. The general formula of this series is CnH2n. In naming these compounds, the suffix of the alkane is replaced by -ene. Two examples:
Alkenes have the form CnH2n.
Naming a more complex example is:
The position number and name of the alkyl group are in front of the double-bond position number. The alkyl group above is an ethyl group. It is on the second carbon atom of the parent hydrocarbon.
The name is: 2-ethyl-1-pentene
If the double bond occurs on an interior carbon, the chain is numbered so that the position of the double bond is designated by the lowest possible number assigned to the first doubly bonded carbon. For example:
The bonding is more complex in the double covalent bond than in the single bonds in the molecule. Using the orbital pictures of the atom, we can show this as follows:
The two p lobes attached above and below constitute one bond called a pi (π) bond.
The sp2 orbital bonds between the carbons and with each hydrogen are referred to as sigma (σ) bonds.
Alkyne Series (Unsaturated)
The alkyne series has a triple covalent bond between two adjacent carbons. The general formula of this series is CnH2n-2. In naming these compounds, the alkane suffix is replaced by -yne. Two examples:
Alkynes have the form CnH2n – 2.
The orbital structure of ethyne can be shown as follows:
The bonds formed by the p orbitals and the one bond between the sp orbitals make up the triple bond.
Notice that pi bonds are between p orbitals and that sigma bonds are between s and p orbitals.
The preceding examples show only one triple bond. If there is more than one triple bond, modify the suffix to indicate the number of triple bonds. For example, 2 would be a diyne, 3 would be a triyne, and so on. Next add the names of the alkyl groups if they are attached. Number the carbon atoms in the chain so that the first carbon atom in the triple bond nearest the end of the chain has the lowest number. If numbering from both ends gives the same positions for two triple bonds, then number from the end nearest the first alkyl group. Then, place the position numbers of the triple bonds immediately before the name of the parent hydrocarbon alkyne and place the alkyl group position numbers immediately before the name of the corresponding alkyl group.
Two more examples of alkynes are:
Naming a more complex example is:
The position number and the name of the alkyl group are placed in front of the double-bond position number. The alkyl group above is an ethyl group. It is on the second carbon atom of the parent hydrocarbon.
The name is: 2-ethyl-1-pentyne
The aromatic compounds are unsaturated ring structures. The basic formula of this series is CnH2n-6, and the simplest compound is benzene (C6H6). The benzene structure is a resonance structure that is represented like this:
Note: The carbon-to-carbon bonds are neither single nor double bonds but hybrid bonds. This structural representation is called resonance structures.
C6H6, benzene is the simplest aromatic compound.
The benzene resonance structure can also be shown like this:
The orbital structure can be represented like this:
Most of the aromatics have an aroma, thus the name “aromatic.”
The C6H5 group is a substituent called phenyl. This is the benzene structure with one hydrogen missing. If the phenyl substituent adds a methyl group, the compound is called toluene or methyl benzene.
Two other members of the benzene series and their structures:
The IUPAC system of naming benzene derivatives, as with chain compounds, involves numbering the carbon atoms in the ring in order to pinpoint the locations of the side chains. However, if only two groups are substituted in the benzene ring, the compound formed will be a benzene derivative having three possible isomeric forms. In such cases, the prefixes ortho-, meta-, and para-, abbreviated as o-, m, and p-, may be used to name the isomers. In the ortho- structure, the two substituted groups are located on adjacent carbon atoms. In the meta- structure, they are separated by one carbon atom. In the para- structure, they are separated by two carbon atoms.
Many of the chain hydrocarbons can have the same formula, but their structures may differ. For example, butane is the first compound that can have two different structures or isomers for the same formula.
This isomerization can be shown by the following equation:
REMEMBER Isomers have the same formula but different structures.
The isomers have different properties, both physical and chemical, from those of hydrocarbons with the normal structure.