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Alkenes & Alkynes. Ethylene (H 2 C = CH 2 ) Ethylene, or ethene, is one of the most valuable products obtained from petroleum refining. Ethylene is the starting material for polyethylene which is found in a variety of commercial products including milk bottles and plastic bags.
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Ethylene (H2C = CH2) • Ethylene, or ethene, is one of the most valuable products obtained from petroleum refining. • Ethylene is the starting material for polyethylene which is found in a variety of commercial products including milk bottles and plastic bags. • Ethylene is the simplest member of the alkene family which consist of hydrocarbon containing one carbon-carbon double bonds.
The Lewis dot representation for ethylene shows the bond resulting from sharing of two pairs of electrons between the two carbon atoms. The alkenes have the general formula[CnH2n].
Butene Butene (C4H8) can exist as three constitutional isomers. Two isomers (1-butene and 2-butene), differ only in the position of the double bond. The third butene isomer has a different carbon skeleton and is named as a 2-methyl-1-propene derivative following the IUPAC rules.
Two forms of 2-butene are possible depending on the relative position of the methyl groups. These two forms are geometric isomers. • The isomer in which the two methyl groups are on opposite sides of the double bond is called trans-2-butene. The other isomer with the methyl groups on the same side of the double bond is cis-2-butene.
What can we say about the relative stability of the four isomers of butene? We have seen that heats of combustion provide a measure of stability. The higher the heat released when a mole of a substance is burned, the less stable it is.(Trans alkenes are more stable than cis alkenes?)
Chemical Reactions of Alkenes: Reaction Map: A concept map showing some of the important reactions of alkenes reveals the two basic types of reactions of alkenes - addition and oxidation.
Organic Nomenclature These rules were developed by the International Union of Pure and Applied Chemistry (IUPAC). IUPAC names are written in the form:
IUPAC Rules for Alkenes: • I. Identify the parent (main) hydrocarbon • Find the longest continuous chain of carbon atoms containing the double bond. The name of the straight-chain alkane with that number of carbon atoms is the parent name. Designate the double bond by changing the family name (suffix) from -ane to -ene. • II. Number the parent chain • Number the parent chain starting from the end nearer the double bond. • If the double bond is the same distance from both ends, begin with the end nearer the first branch point. • III. Assign (detect) each substituent name and number • Use the same rules as for branched-chain alkanes.
IV. Form (write) a single word name for the alkene • Combine the numbers and names of all substituents with the parent name to form one word. Use hyphens (-) to separate numbers from names. Use commas (,) to separate numbers. • If two or more substituents are attached to the parent chain, write them in alphabetical order. If two or more substituents are identical, use prefixes (di-, tri-, tetra-, penta-, etc.). Each substituent must have a number, even if the numbers must be repeated. • Indicate the position of the double bond by giving the number of the first carbon of the double bond before the -ene ending. If more than one double bond is present use the suffixes -diene, triene, etc.
Exercise Name the following compound using the IUPAC system. Draw the structures represented and give the correct names. (a) 2-Methyl-4-hexene (b) 5, 5-Dimethyl-3-hexene (c) 2-Butyl-1-propene
Alkynes: • Alkynes are hydrocarbons containing carbon-carbon triple bond. They exhibit neither geometric nor optical isomerism. The simplest alkyne is ethyne ,commonly known as acetylene: • Multiple Bonds Between Carbon Atoms: • In alkenes pairs of atoms can share between themselves more than just a single pair of electrons. Alkynes take this sharing a step further than alkenes, sharing three electron pairs between carbons instead of just two.
Two π Bonds: • Alkenes are molecules that consist of carbon and hydrogen atoms where one or more pairs of carbon atoms participate in a double bond, which consists of one sigma (σ) and one pi (π) bond. Alkynes are also molecules consisting of carbon and hydrogen atoms, but instead of forming a double bond with only one sigma (σ) and one pi (π) bond, the alkyne has at least one pair of carbon atoms who have one σ and two π bonds ( triple bond)
The carbon-carbon triple bond, then, is a bond in which the carbon atoms share an (s) and two (p) orbitals to form just one σ and two (π) bonds between them. • This results in a linear molecule with a bond angle of about 180ﾟ. Since we know that double bonds are shorter than single covalent bonds, it would be logical to predict that the triple bond would be shorter. The triple bond’s length, 1.20A°, is shorter than that of ethane and ethene's 1.54 and 1.34 angstroms, respectively.
Nomenclature: • The alkynes are named according to two systems. In one, they are considered to be derived from acetylene by replacement of one or both hydrogen atoms by alkyl groups. • For more complicated alkynes the IUPAC names are used. The rules are exactly the same as for the naming of alkenes, except that the ending( -ynereplaces –ene). • The parent structure is the longest continuous chain that contains the triple bond, and the positions both of substituents and of the triple bond are indicated by numbers.
The triple bond is given the number of the first triply-bonded carbon encountered, starting from the end of the chain nearest the triple bond.
Physical properties of alkynes: • Alkynes have low polarity, the alkynes have physical properties that are essentially the same as those of the alkanes and alkenes. They are insoluble in water but quite soluble in the usual organic solvents of low polarity(ether, benzene, carbon tetrachloride). • They are less dense than water. Their boiling points show the usual increase with increasing carbon number.
Reaction of alkynes: • Alkyne chemistry is the chemistry of the carbon-carbon triple bond. Like alkenes, alkynes undergo electrophilic addition, and for the same reason: availability of the loosely π electrons. • Addition Reactions:
2- Addition of halogens 3- Addition of hydrogen halides
Combustion: • Alkynes burn in air with yellow flame. Alkenes also burn yellow, while alkanes burn with blue flames. Acetylene burns with large amounts of heat, and is used for welding metals together.
