The Unsaturated Hydrocarbons: Alkenes and Alkynes

Alkenes and Alkynes: Structure and Physical Properties
An unsaturated hydrocarbon is a hydrocarbon containing at least one double or triple bond.
An alkene is a hydrocarbon containing double bonds. The general formula of an alkene is CnH2n.
An alkyne is a hydrocarbon containing triple bonds. The general formula of an alkyne is CnH2n-2.

The term "saturated" is used to refer to a compound in which all carbon-carbon bonds are single bonds and every carbon atom is connected to a different atom. A saturated hydrocarbon will contain all the hydrogen atoms possible according to the alkane general formula CnH2n+2.

Ethane, C2H6, is an example of a saturated hydrocarbon.

Other examples of saturated compounds are octane, C8H18, and diethyl ether, C4H10O.

The term "unsaturated" is used to designate a compound which contains double or triple bonds and therefore not every carbon is bonded to a different atom.

Ethene, C2H4, is an example of an unsaturated hydrocarbon.

Other examples of unsaturated compounds are benzene, C6H6, and acetic acid, C2H4O2.

All bond angles about the carbon-carbon double bond are 120o. All atoms bonded to the double bonded carbon atoms lie in the same plane.

Carbon-carbon triple bond angles are 180o. Thus, all atoms bonded to the triple bonded carbons are in a straight line.

Like alkanes, both alkenes and alkynes are nonpolar and have relatively low melting points and boiling points. Alkynes generally have slightly higher boiling points than alkenes.

Alkanes have the general molecular formula CnH2n+2. Alkanes are saturated hydrocarbons because each member of the family has the maximum number of hydrogen atoms per carbon atom in its molecular formula.

Alkenes have the general formula CnH2n, and are examples of unsaturated hydrocarbons. The presence of a double bond compound's structure causes there to be two fewer hydrogen atoms in the molecule than there are in an alkane with the same number of carbon atoms.

A molecule with 1 degree of unsaturation (hydrogen deficiency index, HDI) could be related to a ring or a double bond. Likewise an HDI of 2 could be 1 ring and 1 double bond, two double bonds, or two rings.

Calculating a Molecule's Degree of Unsaturation (HDI)
1. Calculate the number of hydrogens for the parent alkane, CnH2n +2.
2.  Adjust the number of hydrogens in the target compound for any heteroatoms.
        a. Add 1 to the target compound for each halogen.
        b. Ignore oxygen in the target.
        c. Subtract 1 from the target compound for each nitrogen.
3. Subtract actual hydrogens from parent hydrogens.
4. Divide the difference by 2.
Molecule Step 1
CnH2n +2
Step 2
Step 3
Step 4
14 6 14 - 6 = 8 8/2 = 4 4
12 5–1 = 4 12 - 4 = 8 8/2 = 4 4
6 7–1 = 6 6 – 6 = 0   0
10 10 10-10 = 0   0
Chlorobenzoic acid C7H5O2Cl 16 5+1=6 16-6 = 10 10/2=5 5

Alkenes and Alkynes: Nomenclature
1. Find the longest carbon chain containing the double bond. This defines the parent compound.

2. Number the carbons from the end that places the double bond between the smallest possible numbers. Designate the location of the double bond with the lower of these two numbers.

3. If enough information is available about the molecule to decide if it is cis or trans, make this designation.

4. Name substituted alkyl groups, as was done for the alkanes.

5. Replace the -ane ending of the parent alkane with -ene.

Polyunsaturated alkenes are alkenes with more than one double bound.
Number of double bonds is indicated using the prefixes di-, tri-, tetra-, and so on.


Naming the Alkynes
1.   Same as alkenes and alkanes.
2.   -yne suffix.

Name = substituents & locations + bond location + stem + -yne
ethyne (acetylene)

Geometric Isomers of Alkenes: Cis and Trans Relationships
Geometric isomers are isomers that owe their existence to hindered rotation about double bonds.
Cis, trans isomerism is applicable for disubstituted alkenes. Disubstituted means that two substituents other than hydrogen are bonded to the double-bond carbons.
Disubstituted alkenes with substituents on the same side of the double bond are referred to as: cis- (Latin: on this side).
Disubstituted alkenes with substituents on the opposite side of the double bond are referred to as: trans- (Latin: across)

Cis-trans isomerism is not limited to disubstituted alkenes. It can occur whenever both of the double-bond carbons are attached to two different groups.

If one of the double-bond carbons is attached to two identical groups, however, then cis-trans isomerism is not possible.

Reactions Involving Alkenes
Alkenes are more reactive than alkanes due to the exposed pi-bonding electrons.

There are four types of common rxns:
1.   Combustion Reactions
2.   Addition Reactions
3.   Oxidation Reactions of Alkenes
4.   Polymerization of Alkenes

1.  Combustion Reactions
    CnH2n + 1.5nO2 ---> n CO2 + n H2O
    C2H4 + 3 O2 ---> 2 CO2 + 2 H2O
    C3H6 + 4.5 O2 ---> 3 CO2 + 3 H2O
As with alkanes, combustion of alkenes with insufficient oxygen also produces CO.

2.  Addition Reactions
Addition reactions can be divided into two classes:
A.  Symmetrical reactants, where the same substituent is added to each carbon of the double bound.

Symmetrical A-A = H2 , or halogen (Cl2 , Br2)
B.  Nonsymmetrical reactant, where a different substituent is added to each carbon of the double bound.

Nonsymmetrical A-B = HCl or H2O
Note about writing equations in organic chemistry
Equations show: 1) Reactants, 2) Conditions, and 3) Products
A + B -------------> C


A -----------> C

2a.  Addition of Hydrogen = Hydrogenation

Hydrogenation generally requires a catalyst such as Pt, Pd and may also require heat and / or pressure.

Hydrogenation is used to convert liquid oils to solid fats. As the number of double bonds is reduced, the melting point of the fat increases. This type of reaction is called hardening. Pt is a common catalyst.

2 b. Addition of Halogens = Halogenation

The double bond reacts rapidly at room temp with either Cl2 or Br2. No catalyst is needed. F>> Cl > Br >>> I (iodine hardly reacts, fluorine reacts explosively).

Halogenation can be a qualitative test.
Suspect compound + dilute solution of Br2. If red color disappears then the compound is probably an alkene.

2 c. Addition of water = Hydration
In the presence of acid, water adds to an alkene double bond.

Addition of water to an alkene follows Markovnikov’s rule.
Markovnikov's Rule: THEM THAT HAS GETS!
When an unsymmetrical molecule such as HCl or H2O adds to a double bond, the double-bonded carbon with the most hydrogens gets the additional hydrogen.

2d.  Addition of Hydrogen Halides = Hydrohalogenation

Addition of a hydrogen halide (HX, X = Br, Cl, or I) to an alkene also follows Markovnikov’s rule. Them that has gets!

3.   Oxidation Reactions of Alkenes
Oxidation can be defined as either  a) the addition of oxygen to a molecule, or  b) tThe removal of hydrogen from a molecule.

Common oxidizing agents: oxygen, O2, ozone, O3, potassium permanganate, KMnO4, potassium dichromate, K2Cr2O7.

Reaction of an alkene with dilute aqueous solution of KMnO4 produces a dihydroxy alcohol.
Ethylene + KMnO4 -----> CH2(OH)CH2OH [ethylene glycol - used in antifreeze]

Oxidation of an alkene with potassium permanganate, KMnO4, can be a qualitative test. Suspect compound + aqueous solution of KMnO4. If purple color disappears then the compound is probably an alkene.

4. Polymerization of Alkenes
Certain alkenes undergo self-addition reactions in the presence of specific catalysts to produce molecules called polymers. A polymer is a very large molecule (macromolecule) made up of repeating units.

The reaction involves double bonds being converted to single bonds as hundreds or thousands of molecules bond and form long chains. The large number of molecules bonding is represented by "n" in the following example polymerization of ethene to from polyethylene.

                        heat, pressure
n CH2=CH2 -------------------->    (-CH2-CH2-)n
ethene            catalysts                polyethylene

The word "polymer" comes from the Greek "poly" (many) and "mer" (parts). An addition polymer is a polymer formed by linking together many alkene molecules through addition reactions. The monomer is the starting material that becomes the repeating units of polymers. It is not possible to give an exact formula for a polymer produced by a polymerization reaction because the individual polymer molecules vary in size. Polymers made from alkenes result in a very long-chain alkane. As a result, it has the chemical inertness of alkanes.

Examples of Addition Polymers
Chemical and
trade name
monomer polymer use
polyethylene CH2=CH2
(-CH2-CH2-)n bottles, bags film
polytetrafluoroethylene (Teflon) CF2=CF2
(-CF2-CF2-)n pan coatings, plumber's tape, heart valves

Aromatic Hydrocarbons
The most common aromatic compound is benzene, C6H6.

The benzene ring consists of six coplanar carbon atoms bonded by alternating double and single bonds. The bond angles about the atoms is 120o.

The double and single bonds can be thought of as alternating positions between the carbon atoms very rapidly. The result is actually not alternating double and single bonds but what is known as a "hybrid" structure.

Nomenclature of Compounds with the Benzene Ring
A common method of nomenclature uses benzene as the parent compound and the name of any atom or groups bonded (monosubstituted) to the ring as a prefix. The names are written as one word with no spaces.

Other aromatic compounds have common names.

When two groups (Y) are present on the ring, three possible orientations exist.

These molecules can be named by either the I.U.P.A.C. Nomenclature System or the common system of nomenclature. Common system uses the term ortho, meta and para.

Groups on adjacent carbon atoms Groups separated by one carbon atom Groups separated by two carbon atoms
ortho (o) meta (m) para (p)
1,2- 1,3 1,4

When there are two or more substituents, some specification of position is required. The numbering system is straightforward. (i.e., clockwise with the #1 C in the 12 o'clock position.) Examples are: 2-ethyltoluene or o-ethyltoluene and 4-ethyltoluene or p-ethyltoluene .

If three or more groups are attached to the benzene ring, numbers must by used to describe their location. The names are given in alphabetical order.

The phenyl group is derived by removing one hydrogen from the benzene ring.

Toluene (methylbenzene) = Ph-CH3
Phenol (hydroxybenzene) = Ph-OH
Anisole (methoxybenzene) = Ph-OCH3
Aniline (benzenamine) = Ph-NH2

Reactions involving benzene
Benzene undergoes substitution reactions.
Ph-H + Y ----> Ph-Y
Rxn Type Substituent (Y) Condition / Catalyst
Halogenation -Cl, or -Br FeX3 (FeCl3 or FeBr3)
Sulfonation -SO3H SO3 / conc. H2SO4
Nitration -NO2 HNO3 / conc. H2SO4 & Heat

Polynuclear aromatic hydrocarbons (PAH) (Fused-Ring Aromatic Systems)

Naphthalene (major ingredient of moth balls)
Benzopyrene (cigarette smoke, charcoal-broiled meat, automobile exhaust)

Heterocyclic Aromatic Compounds
Heterocyclic aromatic compounds have a least one atom other than carbon (heteroatom) in the aromatic ring.