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• unsaturated hydrocarbon

Physical Properties

Spectroscopic Properties

Compared to saturated hydrocarbons, the unsaturated hydrocarbons not only contains the C-C bonds and C-H bonds, but also have C=C double bonds and C≡C triple bonds. As a result, the spectrum will also contain characteristics of these πぱい bondings. Similar as alkanes, the spectroscopy of unsaturated hydrocarbons will not shows the characteristics of other functional groups such as alcohol(-OH) and carboxylic acid(-COOH).

Infrared Spectroscopy

The stretching of C=C bond will give an IR absorption peak at 1670-1600cm^-1, while the bending of C=C bond absorbs between 1000-650 cm^-1 wavelength. The stretching of C≡C bond absorbs 2100-2140 cm^-1(monosubstituted) and 2190-2260 cm^-1(disubstituted). The strength of these absorption peaks varies with the place and number of the double or triple bonds.

Because of the delocalized πぱい electrons in aromatic groups, the bending of C=C bond in these groups usually absorbs between 1500-1700 cm^-1.

At the mean time, the absorption peaks of C-H and C-C bond, which are shared with the saturated hydrocarbons, also shows in the IR spectrum of unsaturated hydrocarbons.

NMR Spectroscopy

In ¹H NMR spectroscopy, the hydrogen bonded to the carbon adjacent to double bonds will give a δでるた_H of 4.5-6.5 ppm.The double bond will also deshield the hydrogen attatched to the carbons adjacent to sp² carbons, and this generates δでるた_H=1.6-2.7 ppm peaks. Aromatic groups will have δでるた_H=6.5-8.5 ppm peaks. Since the πぱい bondings will make cis/trans isomers, the unsaturated hydrocarbon isomers will appear differently due to different J-coupling effect. Cis vicinal hydrogens will have coupling constants in the range of 6-14 Hz, whereas the trans will have coupling constants of 11-18 Hzへるつ.

In ¹³C NMR spectroscopy, compared to the saturated hydrocarbons, the double and triple bonds also deshiled the carbons, making them have low field shift. C=C double bonds usually have chemical shift of about 100-170 ppm.

Chemical Properties

Combustion

Like most other hydrocarbons, unsaturated hydrocarbons can go under combustion reactions that produces carbon dioxide and water in complete combustion. The reaction equation is:

• C_xH_y + y+2x/2O₂ → yH₂O + xCO₂

In the absence of oxygen, the combustion will turn into incomplete combustion and produce carbon monoxide and carbon.

The unsaturated hydrocarbons will produce incomplete combustion product more easily than saturated ones. As a result, the combustion of unsaturated hydrocarbons usually have yellow flame, different from the blue flame of the saturated ones. This indicates unsaturated hydrocarbon combustion will involve multi-step mechanisms, and the burning of carbon gives the yellow flame color.

Since unsaturated hydrocarbons have less hydrogen content, it will produce less water and decrease the flame moisture, as well as decrease the oxygen use. Acetylene(ethyne), for example, can be uesd as fuel.

Acetylene fuel container/burner as used in the island of Bali

Compared to the single σしぐま C-C bonds in the saturated hydrocarbons, the unsaturated ones have electron density in the πぱい bonds, which do not have much electron density overlapping as the σしぐま. As a result, the chemical energy stored in one double bond is less than in two single bonds. Thus, the combustion of unsaturated hydrocarbons, which breakes the carbon-carbon bonds to release energy, release less energy than burning same molarity of saturated ones with same number of carbons. This trend can be clearly seen in the list of standard enthalpy of combustion of hydrocarbons.

Number of Carbon	Substance	Type	Formula	Hcø(kJ/mol)
2	ethane	saturated	C2H6	-1559.7
	ethene	unsaturated	C2H4	-1410.8
	ethyne	unsaturated	C2H2	-1300.8
3	propane	saturated	CH3CH2CH3	-2219.2
	propene	unsaturated	CH3CH=CH2	-2058.1
	propyne	unsaturated	CH3C≡CH	-1938.7
4	butane	saturated	CH3CH2CH2CH3	-2876.5
	but-1-ene	unsaturated	CH2=CH-CH2CH3	-2716.8
	but-1-yne	unsaturated	CH≡C-CH2CH3	-2596.6

Electrophilic Addition

The double or triple bonds that must present in unsaturated hydrocarbons provide high electron density that make the molecules become perfect spots for electrophilic addition reactions. In this kind of reaction, one πぱい bond between carbons will break into 2 separate σしぐま bonds between each carbon and the added group. A carbocation intermediate is usually involved in the mechanism.

typical electrophilic addition reaction of ethene

Hydrogenation

Main article: hydrogenation

Hydrogenation is the electrophilic addition of hydrogen gas to unsaturated hydrocarbon. The result will be a more saturated hydrocarbon, but not necessarily become a saturated one. For instance, semihydrogenation of an alkyne may form an alkene. Nonetheless, the total number of πぱい bond must decrease in the process. The πぱい carbon-carbon bond is also necessary for this process.

The reaction equation of hydrogenation of ethene to form ethane is:

• H₂C=CH₂ + H₂→H₃C-CH₃

The hydrogenation reaction usually requires catalysts to increase its rate.

The total number of hydrogen that can be added to an unsaturated hydrocarbon depends on its degree of unsaturation. An unsaturated hydrocarbon with formula of C_XH_Y can have 2X+2-Y hydrogen atoms at most added to it. This will make the molecule become saturated.

Halogenation

Main article: halogenation

Similar as hydrogen, the heterolysis of halogen(X₂) will produce a electrophilic X⁺ ion, after which it will be attacked by the electron on the πぱい bond. Different from hydrogen, halogenation will produce halonium ions as intermediate instead of carbocations in most other cases. The halonium cation leaves limited space for the X^- ion to attack and will only turn into a trans product. The net result of halogenation is decrease of one πぱい bond and increase two carbon-halogen σしぐま bonds on the 2 carbons.

Structure of a bromonium ion

The reaction equation for bromine addition of ethene, for example, is:

• H₂C=CH₂ + Br₂→H₂CBr-CH₂Br (trans)

Bromine test is used to test the saturation of hydrocarbons. The test involves the addition of bromine water to the unknown hydrocarbon; If the bromine water is decolourized by the hydrocarbon, which is due to halogenation reaction, it can then be concluded that the hydrocarbon is unsaturated. If it is not decolourized, then it is saturated.

The bromine test can also determine the degree of unsaturation for unsaturated hydrocarbons. Bromine number is defined as gram of bromine able to react with 100g of product. Similar as hydrogenation, the halogenation of bromine is also depend on the number of πぱい bond. A higher bromine number indicates higher degree of unsaturation.

Hydration

Main article: hydration

The πぱい bond of unsaturated hydrocarbons are also ready to accept H⁺ and OH^- from water. The reaction usually involves strong acid as catalyst. That is because the first step of mechanism of hydration involves the πぱい bond deprotonate a H⁺ from the strong acid to form a carbocation. The net result of the reaction will be an alcohol.

The reaction equation for hydration of ethene is:

• H₂C=CH₂ + H₂O→H₃C-CH₂OH

The πぱい bonds in triple bond are also able to go under hydration in acidic condition and form enols. However, the enol will not be a product but an intermediate, and the final product will be a ketone. The enol intermediate goes under tautomerization and form the more stable ketone.

The reaction equation of hydration of ethyne to form acetaldehyde is:

• HC≡CH + H₂O → H₂C=CH-OH
• H₂C=CH-OH ⇌ H₃C-CHO

Hydrohalogenation

Main article: hydrohalogenation

The hydrohalogenation involves addition of H-X to unsaturated hydrocarbons. This will decrease one πぱい C=C bond and result in 2 C-H and C-X σしぐま bonds with 2 separate carbons. The formation of the intermediate carbocation is selective and follows the Markovnikov's rule. The hydrohalogenation of alkene will result in haloalkane, and hydrohalogenation of alkyne will result in vinyl halide. The hydrohalogenation of alkyne is much slower than the alkene.

The reaction equation of HBr addition to ethene is:

• H₂C=CH₂ + HBr→H₃C-CH₂Br