Chemistry 110 Bettelheim, Brown, Campbell & Farrell Ninth Edition Introduction to General, Organic and Biochemistry Chapter 12 Alkenes & Alkynes
Chapter 12 Alkenes are hydrocarbons which have one or more carbon-carbon double bonds. VSEPR angles 120E VSEPR angles 180E Alkynes are hydrocarbons which have one or more carbon-carbon triple bonds. These have little import to biochemistry and will not be studied further in this course. Fig. 12.UN, p.314
Physical Properties of Alkenes and Alkynes Alkenes and alkynes are nonpolar compounds. The only attractive forces between their molecules are London dispersion forces. Their physical properties are similar to those of alkanes with the same carbon skeletons. Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids. Alkenes and alkynes that are liquid or solid at room temperature have densities less than 1 g/ml; they float on water.
Geometric (cis-trans) Isomerism in Butenes C 3 C 2 C C 1-Butene No geometrical isomers possible. Different Connectivity 3 C C C 3 C Methylpropene C 3 C 3 C 3 Same connectivity but C C C C different geometry. C 3 cis-2-butene trans-2-butene geometric isomers
1 Geometric (cis-trans) Isomerism C 3 C C C 2 C 3 2 3 The groups can be different! 4 5 1 C 3 C C 2 3 C 2 C 3 4 5 cis-2-pentene trans-2-pentene geometric isomers Cl C 2 C 3 C C cis-1-chloro-1-butene Cl C C C 2 C 3 trans-1-chloro-1-butene
IUPAC Systematic Naming of Alkenes Longest continuous carbon chain containing the double bond = Parent Alkene (PA; even if a longer chain without the double bond exists!); assign numbers counting from the end giving the first carbon of the double bond the lowest number. Locate any chain branching off PA, name as alkyl group. For multiple branching Locate identical alkyl groups, indicate their number using bi, di (2); tri (3); tetra (4) Locate different alkyl groups. Construct the name by locating the double bond with a number preceeding the PA separated by hyphens, place the alkyl groups in alphabetical order (ignoring Greek prefixes, dimethyl = m) preceded by locator numbers separated by hyphens from words and commas from other numbers, a locator number for each alkyl group, i. e. 2,2-dimethyl. 1 2 C 3 CC 3 C 3 CCC 2 C 2 CC 3 C 3 3 4 5 6 7 3-isopropyl-2,6- dimethyl-2-heptene C 3
IUPAC Systematic Naming of cycloalkenes Largest carbon ring containing the double bond = Parent cycloalkene (PCA) use cyclo+(name of straight chain having same number of carbon atoms as the ring). Count the carbon atoms of the double bond 1 and 2 in the direction giving the nearest substituent the lowest number. Locate any chain branching off PCA, name as alkyl group. For multiple branching Locate identical alkyl groups and indicate their number using bi, di (2); tri (3); tetra (4) Locate different alkyl groups. Construct the name by placing the alkyl groups in alphabetical order (ignoring Greek prefixes, dimethyl = m) preceded by locator number separated by hyphens from words and commas from numbers. There should be a locator number for each alkyl group. C(C 3 ) 2 2 3 4 5-ethyl-3-isopropylcyclohexene 1 6 5 C 2 C 3
Addition Reactions to the Double Bond Addition of Cl 2 and Br 2 Addition takes place readily at room temperature reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent Br Br C 3 C= CC + 3 Br 2 C C 2 Cl 3 C-CC 3 2 2-Butene 2,3-Dibromobutane Br + Br 2 C 2 Cl 2 Br Cyclohexene 1,2-Dibromocyclohexane addition of Br 2 is a useful qualitative test for the presence of a carbon-carbon double bond Br 2 has a deep red color; dibromoalkanes are colorless
Addition Reactions to the Double Bond C C + Cl C C ethene, ethylene Cl chloroethane C 3 C C C 3 cis-2-butene 2 SO 4 + O C 3 C C C 3 O 2-butanol
Addition Reactions to the Double Bond O O C C + S C C ethene O O O O ethyl hydrogen sulfate S O O C 3 C 2 C C 2 1-butene + O C 3 C 2 C C 2 O 2-butanol
Unsymmetrical Addition Reactions Markovnikov s Rule The carbon having the most hydrogen atoms gets the hydrogen atom. C 3 C 2 C C 2 1-Butene + Cl C 3 C 2 C C 2 Cl 2-chlorobutane C 3 C 3 C 3 C C 2 + O C 3 C C 2 O Methylpropene 2-methyl-2-propanol
Unsymmetrical Addition Reactions Non-Markovnikov 3 C C 2 C 3 C C Br 3 C C 2 C 3 3-bromopentane 50% C C + Br cis-2-pentene 3 C C 2 C 3 C C Br 2-bromopentane 50%
ow Addition Reactions Occur Positively Polarized ydrogen Atoms in molecules react with alkenes to form Carbocations G + C 3 C C 2 C 3 C C 3 + G Carbocation Stability: tertiary > secondary > primary > methyl C 3 C C 3 C 3 C 2 C 2 secondary >> primary cation
Carbocations Lewis Structure C C C G G C C C The center carbon has only 3 bonds and an incomplete octet of electrons. Such molecules are strong Lewis Acids and are readily attacked by molecules having lone pairs of electrons Lewis Bases
Carbocations Lewis Structure C C C O O C C C + The water molecule (and many others) is too weak a Bronsted acid to protonate an alkene and make a carbocation. A strong acid catalyst is added to form the carbocation. The water molecule then donates its lone pair (and the O bearing it) to the carbocation forming an oxonium ion. It then loses a proton to form the alcohol.
Carbocation Stability Directs Addition To Double Bonds 3 C C 3 C 3 C C 2 3 C C C 3 C 3 C 2 C C 2 C 3 C 2 C C 3 C 2 C 2 C 2 C 3
Polymers Can Be Made By Carbocation Addition to Alkenes C 3 C 3 C C 3 C C 2 C 3 C 3 C C 3 C 3 C 3 C C 2 + C C 2 CC 3 C 3 C 3 C 3 C 3 C 3 + C C 2 CC 3 C C 2 (CC 2 ) 2 C C 2 repeating unit