Alkenes: Structure and Reactivity

Calculating Degree of UnsaturationCalculating 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 CnH2n+2

• Alkene has fewer hydrogens than an alkane with the same number of carbons —CnH2n because of double bond

• Each ring or multiple bond replaces 2 H's

Example: CExample: C66HH1010

• Saturated is C6H14

– 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 ElementsDegree of Unsaturation With Other Elements

• Organohalogens (X: F, Cl, Br, I)• Halogen replaces hydrogen

• C4H6Br2 and C4H8 have one degree of unsaturation

Degree of Unsaturation (Continued)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 CompoundsOrganonitrogen 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) + 2 + (Number of N)

Next, subtract the actual number of H then divide by 2

Cis-Trans Cis-Trans Isomerism in AlkenesIsomerism in Alkenes

• Carbon atoms in a double bond are sp2-hybridized– Three equivalent orbitals at 120º separation in plane– Fourth orbital is atomic p orbital

• Combination of electrons in two sp2 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 Cis-Trans Isomerism in Alkenes (Continued)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 Cis-Trans Isomerism in Alkenes (Continued)Isomerism in Alkenes (Continued)

Cis-Trans Cis-Trans Isomerism in Alkenes (Continued)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 Alkene Stereochemistry and the E,ZE,Z Designation 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,ZE,Z Stereochemical Nomenclature 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 RulesCahn-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 RulesCahn-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 RulesCahn-Ingold-Prelog Rules

Stability of AlkenesStability of Alkenes

• Cis alkenes are more polar, less stable than trans alkenes• Compare heat given off on hydrogenation: Ho

• Less stable isomer is higher in energy– And gives off more heat– hyperconjugation of R groups stabilize the pi bond

Effects of Increasing Alkene SubstitutionEffects 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.

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Stability of Alkenes (Continued): HyperconjugationStability of Alkenes (Continued): Hyperconjugation

• Electrons in neighboring filled orbital stabilize vacant antibonding orbital – net positive interaction

• Alkyl groups are better than H