Chapter 7Alkenes: Structure and Reactivity

Chapter 7Alkenes: Structure and Reactivity

7.2 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 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: C6H10

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 Elements

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

C4H6Br2 and C4H8 have one degree of unsaturation

Degree of Unsaturation (Continued) Organoxygen compounds (C,H,O) – Oxygen forms 2

bonds these don't affect the formula of equivalent

hydrocarbons May be ignored in calculating degrees of

unsaturation

Organonitrogen Compounds

Nitrogen has three bonds So if it connects where H was, it adds a connection

point Subtract one H for equivalent degree of

unsaturation in hydrocarbon

Count pairs of H's below CnH2n+2

Add number of halogens to number of H's (X equivalent to H)

Ignore oxygens (oxygen links H) Subtract N's - they have two connections

Summary - Degree of Unsaturation

Rotation of bond is prohibitive This prevents rotation about a carbon-carbon double

bond (unlike a carbon-carbon single bond).

Cis-Trans Isomerism in Alkenes

7.6 Stability of Alkenes

Cis alkenes are less stable than trans alkenes

Less stable isomer is higher in energy

Stability of Alkenes (Continued): Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C-C More stable alkene gives off less heat

trans-Butene generates 5 kJ less heat than cis-butene

7.6 Stability of Alkenes

Less stable isomer is higher in energy

tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

Hydrogenation Data Helps to Determine Stability

7.7 Electrophilic Addition of Alkenes

General reaction mechanism of electrophilic addition

Attack on electrophile (such as HBr) by bond of alkene

Produces carbocation and bromide ion

Carbocation is an electrophile, reacting with nucleophilic bromide ion

Two step process First transition state is high energy point First step is slower than second

Electrophilic Addition of Alkenes (Continued): Electrophilic Addition Energy Path

Electrophilic Addition of Alkenes (Continued)

The reaction is successful with HCl and with HI as well as HBr

HI is generated from KI and phosphoric acid

7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule

In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other

If one orientation predominates, the reaction is regioselective Markovnikov observed in the 19th century that in the addition of

HX to alkene, the H attaches to the carbon with more H’s and X attaches to the other end (to the one with more alkyl substituents) This is Markovnikov’s rule

Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not regiospecific

Example of Markovnikov’s Rule

Markovnikov’s Rule (restated)

More highly substituted carbocation forms as intermediate rather than less highly substituted one

Tertiary cations and associated transition states are more stable than primary cations

Markovnikov’s Rule (restated)

Definitions

Regioisomers – two constitutional isomers that could result from an addition reaction.

Regioselective – both regioisomers are formed, but one is formed in preference.

“Regiospecific” – only one regiosisomer forms at the expense of the other.

7.9 Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is

surrounded by only 6 electrons in sp2 orbitals the fourth orbital on carbon is a vacant p-orbital the stability of the carbocation (measured by energy needed to

form it from R-X) is increased by the presence of alkyl substituents

Carbocation Structure and Stability (Continued)

A plot of DH dissociation shows that more highly substitued alkyl halides dissociate more easily than less highly substituted ones

Carbocation Structure and Stability (Continued) A inductive stabilized cation species

Competing Reactions and the Hammond Postulate

Normal Expectation: Faster reaction gives more stable intermediate

Intermediate resembles transition state

7.11 Evidence for the Mechanism of Electrophilic Addition: Carbocation Rearrangments

Carbocations undergo structural rearrangements following set patterns

1,2-H and 1,2-alkyl shifts occur

Goes to give most stable carbocation

Some molecules are have structures that cannot be shown with a single representation

In these cases we draw structures that contribute to the final structure but which differ in the position of the bond(s) or lone pair(s)

Such a structure is delocalized and is represented by resonance forms

The resonance forms are connected by a double-headed arrow

2.4 Resonance

A structure with resonance forms does not alternate between the forms

Instead, it is a hybrid of the resonance forms, so the structure is called a resonance hybrid

For example, benzene (C6H6) has two resonance forms with alternating double and single bonds In the resonance hybrid, the actual structure, all its C-C bonds

are equivalent, midway between double and single

Resonance Hybrids

Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)

Resonance forms differ only in the placement of their or nonbonding electrons

Different resonance forms of a substance do not have to be equivalent

Resonance forms must be valid Lewis structures: the octet rule generally applies

The resonance hybrid is more stable than any individual resonance form would be

2.5 Rules for Resonance Forms

We can imagine that electrons move in pairs to convert from one resonance form to another

A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow

Curved Arrows and Resonance Forms

Any three-atom grouping with a p orbital on each atom has two resonance forms

2.6 Drawing Resonance Forms

Sometimes resonance forms involve different atom types as well as locations

The resulting resonance hybrid has properties associated with both types of contributors

The types may contribute unequally The “enolate” derived from acetone is a good illustration, with

delocalization between carbon and oxygen

Different Atoms in Resonance Forms

The anion derived from 2,4-pentanedione Lone pair of electrons and a formal negative charge on

the central carbon atom, next to a C=O bond on the left and on the right

Three resonance structures result

2,4-Pentanedione