Chapter 7
Alkenes: Structure and Reactivity
Chapter 7
Alkenes: 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
Summary - Degree of Unsaturation
 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
Cis-Trans Isomerism in Alkenes
 Rotation of  bond is prohibitive
 This prevents rotation about a carbon-carbon double
bond (unlike a carbon-carbon single bond).
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
Electrophilic Addition of Alkenes (Continued):
Electrophilic Addition Energy Path
 Two step process
 First transition state is high energy point
 First step is slower than second
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
Example of 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
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 1,2alkyl shifts occur
 Goes to give
most stable
carbocation
2.4 Resonance
 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
Resonance Hybrids
 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
2.5 Rules for Resonance
Forms
 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
Curved Arrows and 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
2.6 Drawing Resonance
Forms
 Any three-atom grouping with a p orbital on
each atom has two resonance forms
Different Atoms in 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
2,4-Pentanedione
 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