Alkenes: Structure and Nomenclature

Alkenes:Structure andNomenclature Chapter 5

Unsaturated Hydrocarbons • Unsaturated hydrocarbon:contains one or more carbon-carbon double or triple bonds • Alkene:contains a carbon-carbon double bond and has the general formula CnH2n • Alkyne:contains a carbon-carbon triple bond and has the general formula CnH2n-2

Unsaturated Hydrocarbons • Arenes:benzene and its derivatives (Ch 21-22) • we do not study arenes until Chapters 21 & 22 • however, we show structural formulas of compounds containing the phenyl group before that time • the phenyl group is not reactive under any of the conditions we describe in Ch 6-20

5.1 A.,B. Structure of Alkenes, Fig. 5.1 • A double bond consists of • one sigma bond formed by the overlap of sp2 hybrid orbitals and one pi bond formed by the overlap of parallel 2p orbitals • the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°

Structure of Alkenes, Fig 5.2 • it takes approximately 264 kJ (63 kcal)/mol to break the pi bond in ethylene; that is, to rotate one carbon by 90° with respect to the other so that there is no overlap between 2p orbitals on adjacent carbons

C.Cis,Trans Isomerism in Alkenes • Cis,trans isomers: isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond

Index of Hydrogen Deficiency • Index of hydrogen deficiency (IHD): the sum of the number of rings and pi bonds in a molecule. This is equivalent to the number of unsaturations in the molecule where a ring is considered to be an unsaturation. • To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds • the molecular formula of the reference hydrocarbon is CnH2n+2 (a saturated hydrocarbon).

) (H - H reference molecule Index of Hydrogen Deficiency • for each atom of a Group 7 element (F, Cl, Br, I), add one H to compound formula or subtract one from the saturated formula. • no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon • for each atom of a Group 5 element (N, P), subtract one hydrogen from the compound formula or add one to the saturated formula. IDH = 2

Index of Hydrogen Deficiency Problem 5.1: isopentyl acetate has a molecular formula of C7H14O2. Calculate its IHD • reference hydrocarbon C7H16 • IHD = (16-14)/2 = 1 Problem 5.2: calculate the IHD for niacin, molecular formula C6H6N2O • reference hydrocarbon C6H16 (remember adding 1 for each N) • IHD = (16 - 6)/2 = 5 O O Isopentyl acetate

5.2 A. IUPAC Nomenclature 1. Number the longest chain of carbon atoms that contains both carbons atoms of the double bond in the direction that gives the carbons of the double bond the lowest numbers 2. Locate the double bond by the number of the first carbon of the double bond encountered 3. Name substituents 4. Number the carbon, locate and name substituents, locate the double bond, and name the main chain

B. Common Names • Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

Common Names • the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups

C. The Cis,Trans System • Configuration is determined by the orientation of atoms of the main chain

The E,Z System • Uses CIP priority rules ( used for R,S in Chapter 3) • if groups of higher priority are on the same side, the configuration is Z (German, zusammen) • if groups of higher priority are on opposite sides, the configuration is E (German, entgegen)

The E,Z System • Example:name each alkene and specify its configuration by the E,Z system

D., E. Cis,Trans Isomerism • Cycloalkenes • in small-ring cycloalkenes, the configuration of the double bond is cis • these rings are not large enough to accommodate a trans double bond

Cis,Trans Isomerism • trans-cyclooctene is the smallest trans cyclooctene that has been prepared in pure form and is stable at room temperature • the cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer • the trans isomer is chiral even though it has no chiral center

F. Dienes, Trienes, and Polyenes • For alkenes containing two or more double bonds, change the infix -en- to -adien-, -atrien-, etc. • those containing several double bonds are often referred more generally as polyenes • following are three dienes

G. Dienes, Trienes, and Polyenes • for alkenes with n double bonds, each of which can show cis,trans isomerism, 2n stereoisomers are possible • example: 22 = 4 cis,trans isomers are possible for 2,4-heptadiene

Dienes, Trienes, and Polyenes • vitamin A, a biologically important compound for which a number of cis,trans isomers is possible • there are four double bonds about which cis,trans isomerism is possible, for 24 = 16 stereoisomers

5.3 Physical Properties • Alkenes are nonpolar compounds • The only attractive forces between their molecules are dispersion forces • The physical properties of alkenes are similar to those of alkanes

5.4 Terpenes • Terpene:a compound whose carbon skeleton can be divided into two or more five carbon isoprene units. Terpenes are classified by their number of carbons, in groups of 10. • A monoterpene has 10 C’s (2 isoprenes). • A diterpene has 20 C’s (4 isoprenes). • A sesquiterpene has 15 C’s (3 isoprenes).

Terpenes • Myrcene, C10H16, a component of bayberry wax and oils of bay and verbena (Fig. 5.3) • Menthol, from peppermint (Fig. 5.4)

Terpenes • -Pinene, from turpentine (Fig. 5.4) • camphor, from the camphor tree (Fig. 5.4)

Fatty Acids • Animal fats and vegetable oils are both triesters of glycerol, hence the name triglyceride • hydrolysis of a triglyceride in base followed by acidification gives glycerol and three fatty acids • fatty acids with no C=C double bonds are called saturated fatty acid • those with one or more C=C double bonds are called unsaturated fatty acids

Fatty Acids • the most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain • the C=C double bonds in almost all naturally occurring fatty acids have a cis configuration • the greater degree of unsaturation, the lower the melting point • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats

Fatty Acids • the four most abundant fatty acids

Fatty Acids • carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation • due to their high degree of order, they pack well and are held together by dispersion forces • as a result both saturated fatty acids and the triglycerides derived from them are solids at room temperature • following is a saturated triglyceride

Fatty Acids • cis double bonds place kinks in the chains of unsaturated fatty acids • unsaturated fatty acids and the triglycerides derived from them do not pack as well in a crystal lattice as their saturated counterparts, and have weaker dispersion forces between their molecules • butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature • salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature

Alkenes: Structure andNomenclature End Chapter 5

Alkenes: Structure and Nomenclature