Alkenes: Structure and Reactivity

Calculating Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for a saturated acyclic compound is C n H 2n+2 Alkene has fewer hydrogens than an alkane with the same number of carbons —C n H 2n because of double bond Each ring or multiple bond replaces 2 H's

Example: C 6 H 10 Saturated is C 6 H 14 – therefore 4 H's are not present This has two degrees of unsaturation – Two double bonds? – or triple bond? – or two rings? – or ring and double bond?

Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I) Halogen replaces hydrogen C 4 H 6 Br 2 and C 4 H 8 have one degree of unsaturation

Degree of Unsaturation (Continued) Organoxygen compounds (C,H,O) – Oxygen forms 2 bonds When 2 bonds are to different things, these don't affect unsaturation, and can be ignored When oxygen forms a double bond to the same ion, assume that 2 H are lost (1 degree of unsaturation)

Organonitrogen Compounds Nitrogen is trivalent, acts like ½ of a carbon To determine degree of unsaturation when N is present, use the following general formula to determine how many H the compound should have: (2 x number of C) (Number of N) Next, subtract the actual number of H then divide by 2

Cis-Trans Isomerism in Alkenes Carbon atoms in a double bond are sp 2 -hybridized – Three equivalent orbitals at 120º separation in plane – Fourth orbital is atomic p orbital Combination of electrons in two sp 2 orbitals of two atoms forms  bond between them Additive interaction of p orbitals creates a  bonding orbital – Subtractive interaction creates a  anti-bonding orbital Occupied  orbital prevents rotation about  -bond Rotation prevented by  bond - high barrier, about 268 kJ/mole in ethylene

Rotation of  bond is prohibitive This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). Creates possible alternative structures Cis-Trans Isomerism in Alkenes (Continued)

the presence of a carbon-carbon double bond can create two possible structures – cis isomer - two similar groups on same side of the double bond – trans isomer - similar groups on opposite sides Each carbon must have two different groups for these isomers to occur Cis-Trans Isomerism in Alkenes (Continued)

 Cis-Trans Isomerization requires that end groups differ in pairs  Bottom pair cannot be superposed without breaking C=C

Alkene Stereochemistry and the E,Z Designation Cis-Trans naming system discussed thus far only works with disubstituted alkenes Tri- and Tetra substituted double bonds require more general method Method referred to as the E,Z system

E,Z Stereochemical Nomenclature Priority rules of Cahn, Ingold, and Prelog Compare where higher priority groups are with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side

Cahn-Ingold-Prelog Rules RULE 1 Must rank atoms that are connected at comparison point Higher atomic number gets higher priority – Br > Cl > S > P > O > N > C > H

RULE 2 If atomic numbers are the same, compare at next connection point at same distance Compare until something has higher atomic number Do not combine – always compare Cahn-Ingold-Prelog Rules

RULE 3 Multiple-bonded atoms are equivalent to the same number of single-bonded atoms Substituent is drawn with connections shown and no double or triple bonds Added atoms are valued with 0 ligands themselves Cahn-Ingold-Prelog Rules

Stability of Alkenes Cis alkenes are more polar, less stable than trans alkenes Compare heat given off on hydrogenation:  H o Less stable isomer is higher in energy – And gives off more heat – hyperconjugation of R groups stabilize the pi bond

Effects of Increasing Alkene Substitution Increasing substitution on alkene carbon atoms increases stability (again, hyperconjugation) This will become very important when we begin to look at possible products resulting from elimination reactions. 17

Stability of Alkenes (Continued): Hyperconjugation Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction Alkyl groups are better than H