Alkanes and Alkenes Lecture Notes PDF

Organic Chemistry 3: Alkanes and Alkenes Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Reactions of Alkanes • Because alkanes are nonpolar, and their covalent bonds are strong, alkanes are relatively unreactive. • The two types of reactions that they do undergo and these are...

Organic Chemistry 3: Alkanes and Alkenes Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Reactions of Alkanes • Because alkanes are nonpolar, and their covalent bonds are strong, alkanes are relatively unreactive. • The two types of reactions that they do undergo and these are: 1. combustion and 2. halogenation (substitution) Combustion In combustion reactions, alkanes react with oxygen to form carbon dioxide, water and heat. + Heat Substitution Reaction If, in a reaction, one atom in a molecule is exchanged by another atom or group of atoms, a substitution reaction has taken place. H H H C H H + Br2 H C Br + HBr H Methylbromide Unsaturated hydrocarbons • • • • Polyethylene plastic bags and bottles Polystyrene Styrofoam cups Plastic wraps Essential oils in plants contain multiple bonds between carbon atoms. • Cosmetics, medicines, flavorings, perfumes • Hydrocarbons also form rings of carbon atoms (aromatics) • Detergents, insecticides, and dyes Bonding in Unsaturated Hydrocarbons The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms. 1. Alkenes contain carbon-carbon double bonds. 2. Alkynes contain carbon-carbon triple bonds. 3. Aromatic compounds contain benzene rings. Nomenclature of Alkenes The general formula for alkenes is: CnH2n Example 2-Pentene IUPAC Rules for Naming Alkenes 1. Select the longest continuous carbon-carbon chain that contains the double bond. 2. Name this parent compound as you would an alkane, but change the –ane ending to –ene. 3 CH3CH2CH3 propane Prefix Number and identity of attached group 2 1 CH3CH=CH2 propene Parent Longest carbon chain Suffix Functional class 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name. CH3CH=CHCH3 2 5 4 2-butene 3 3 2 1 CH3CH2CH2CH=CH2 3 2 1-pentene 1 CH3CH=CH2 Propene (not 1-propene) IUPAC Rules for Naming Alkenes 4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded. H2C CH3 H3C 5 CH 4 H2 C 3 H C 2 4-methyl-1-pentene CH2 1 H3C 6 H2 H2 C C CH 5 4 3 H2 C CH3 H C 2 CH2 1 3-propyl-1-hexene How would we write the structural formula for 4-methyl-2-pentene? CH3 1 H3C H C 2 3 C H CH 4 5 CH3 Draw the structural formula for: 7-methyl-2-octene H C H3C 1 2 3 C H H2 C 4 5 C H2 H2 C 6 7 CH CH3 8 CH3 Octene as it indicates an 8-carbon chain The chain contains a C=C between carbons 2 and 3. There is a –CH3 group on carbon 7 Longest chain containing C=C is 5 carbons. Ethyl group H2 C H3C H2 C 2C 3 4 C H2 CH3 5 1 CH2 Longest chain containing C=C is 5 carbons. 2-ethyl-1-pentene Geometric Isomerism in Alkenes Compounds containing a carbon-carbon double bond (pi bond) have restricted rotation around that double bond. This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism. Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers. They are also called cis-trans isomers. An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it. a a a b 1 2 1 2 C C C C b b cis isomer b a trans isomer Rules for cis-trans isomer 1. Each carbon atom of C=C double bond must have two different groups attached to it. For example groups, a and b are attached to carbon atom 1 and 2 2. At least one attached group between two carbon atoms of C=C (1 and 2) must be same. For example group “a” is same in carbon atom 1 and 2. Group “b” is same in carbon atom 1 and 2. 3. Isomers; cis isomer=group “a” or group “b” on the same side of the double bond, trans isomer=group “a” or group “b” on the opposite side (across the double bond). An alkene does not show cis-trans isomerism if one carbon of the double bond has two identical groups attached to it. H H C H H3C two groups the same C H CH3 C H3C C H Geometric Isomers in Alkenes Cl Cl C H H Cl C C H cis-1,2-dichloroethene (bp = 60.1 C) Cl C H trans-1,2-dichloroethene (bp = 48.4 C) Draw a structure for cis-5-chloro-2-hexene 6 CH3 H3C 1 2C H 4 H2C C 3 5 CH Cl H The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5. Is the compound below the cis or trans isomer? H3C 1 CH3 2 C H 3 C 4 H2C Cis-3-methyl-2-pentene Or Trans-3-methyl-2-pentene 5 CH3 Many compounds have more than one C=C. Compounds with two C=C are called dienes. Compounds with three C=C are called trienes. 1 H2C H C 2 3 C H CH2 4 1,3-butadiene 3 1 2 5 4 7 6 1,3,5-heptatriene Cycloalkenes As the name implies, cycloalkenes are cyclic compounds that contain a C=C in the ring. The carbons of the double bond are assigned numbers 1 and 2. cyclohexene 1 2 2 cyclopentene 1 Cycloalkenes CH3 5 4 CH3 4 1 2 3 1-methylcyclopentene 3 5 2 6 1 CH3 1,3-dimethylcyclohexene EXERCISES Name each compound according to the IUPAC system. Name each compound according to the IUPAC system.