Topic 2: Alkenes: Structure Alkenes are hydrocarbons that contain a carbon-carbon double bond. The word olefin is used as a synonym for alkene, but the term alkene is preferred. Alkenes occur abundantly in nature. Ethylene, for example, is a plant hormone that induces ripening in fruit. Life would be impossible without such alkenes as β-carotene, which contains 11 double bonds. β-carotene is the orange pigment responsible for the color of carrots, serves as a valuable source of vitamin A and is thought to offer some protection against certain types of cancer. β-carotene Additionally, many alkenes serve as the precursors to many industrially important compounds. Approximately 26 million tons of ethylene (CH2=CH2) and 14 million tons of propylene (CH3CH=CH2) are produced each year in the U.S. for use in the synthesis of many important substances.

I. Calculating a Molecule’s Degree of Unsaturation UNSATURATED: Refers to molecules that contain less than the max. number of H atoms Remember that alkanes are saturated, meaning that they contain the max. number of H atoms DEGREE OF UNSATURATION: Each ring and/or double bond corresponds to a loss of 2 H atoms (from the alkane formula CnH2n+2) and = 1° unsaturation Ex. 1° unsaturation 2° unsaturation

[ (2 x # of C + # of N ─ # of halogens ─ # of H ) + 2 ] ÷ 2 The degrees of unsaturation for a molecule can be calculated from the following formula: [ (2 x # of C + # of N ─ # of halogens ─ # of H ) + 2 ] ÷ 2 Keep in mind that this formula will tell you how many degrees of unsaturation are present in a molecule but it will not tell you whether the unsaturation is due to a multiple bond or a ring.

III. Cis – Trans Isomerism in Alkenes * Because rotation around a double C-C bond is impossible without breaking the bond, cis-trans isomerism occurs in alkenes just as it does for cyclic compounds. CIS ISOMERS: Substituents are on the same side Ex. cis-2-butene TRANS ISOMERS: Substituents are on the opposite side Ex. trans-2-butene

If one of the double bonds is attached to two identical groups, cis- trans isomerism cannot occur. These molecules are identical: These molecules are not identical: They are not cis-trans isomers They are cis-trans isomers

IV. Naming Alkanes * Cis-trains isomerism is not limited to disubstituted alkenes. However, when more than two substituents are present, such as in trisubstituted (3 substituents other than H) or tetrasubstituted (4 substituents other than H), the cis-trans naming system fails. A more general method for describing double bond geometry, called the E, Z system, is used. CAHN, INGOLD, and PRELOG : The three scientists who proposed the sequence rules for the E, Z naming system (circa 1966)

Z E CAHN ─ INGOLD ─ PRELOG SEQUENCE RULES The rules are used to assign high and low priority to the groups attached to each carbon. These sequence rules, named after the scientists who proposed them, consider each of the carbon atoms in the double bond separately. If the high priority groups are on the same side of the double bond, the designation Z is used (from the German word zusammen meaning “together”). If the high priority groups are on the opposite side of the double bond, the designation E is used (from the German word entgegen meaning “opposite”). Z means the groups are on “ze zame zide” Z E

Br > Cl > O > N > C > H Rule 1: Considering each of the carbon atoms of the double bond separ- ately, identify the two atoms directly attached and rank them according to atomic number. 35 17 8 7 6 1 Br > Cl > O > N > C > H low high low low high low high high (E)-2-chloro-2-butene (Z)-2-chloro-2-butene

Rule 2: If a decision can’t be reached by ranking the 1st attached atom, then look at the 2nd, 3rd, etc. low high low high low high

Rule 3: Multiple-bonded atoms are equivalent to the same number of single-bonded atoms. is equivalent to

V. Alkene Stability * Although cis-trans interconversion of alkene isomers does not occur spontaneously without breaking the pi bond, it can be made to happen by treating the alkene with a strong acid catalyst. When this is done, many time it is found that one isomer is preferred over another, suggesting that some isomers are more stable than others. H+ Cis (24%) Trans (76%)

STERIC STRAIN: The strain imposed on a molecule when two groups try to occupy the same space bulkier groups = more strain HYDROGENATION REACTION: Reaction in which an alkene or alkyne reacts w/ H2 in the presence of a Pt or Pd catalyst to form an alkane Ex. HEAT OF HYDROGENATION: Amount of heat released (measured as ∆H = change in enthalpy) during a hydrogenation reaction The more heat released upon hydrogenation, the more unstable (more steric strain) the original alkene was

Ex. Cis Isomer Trans Isomer ∆H = -120 kJ ∆H = -115 kJ *less stable *more stable