Chapter 4 Introduction to Alkenes. Structure and Reactivity

Alkenes Hydrocarbons containing one or more carbon-carbon double bonds Sometimes called olefins Classified as unsaturated hydrocarbons Ethylene is the simplest alkene

Physical Properties Very similar to corresponding alkanes: Flammable Non-polar Insoluble in Water Low melting and boiling points Low density compared to water 5

Bond Length Carbon-carbon multiple bonds are shorter than their carbon-carbon single bond counterparts 133.9 pm 153.6 pm

Dipole moments and polarization

Structure and Bonding in Alkenes Carbons involved in the carbon-carbon double bond exhibit trigonal planar geometry

Carbon Hybridization in Alkenes sp2-hybridization  Trigonal planar geometry

sp2 = 33% s character, 66% p character More s character → electrons are held closer to the nucleus

Carbon Hybridization in Alkanes

Carbon-carbon single bond in propene = 150 pm Carbon-carbon single bond in propane = 154 pm Due to sp2 vs. sp3 hybridization Bonds with more s character are shorter

Hybrid Orbital Picture of Ethylene 4.1 Structure and Bonding in Alkenes

s bond: head-to-head overlap of orbitals Cylindrically symmetric

bond: side-to-side overlap of p orbitals

p Orbital System in Ethylene 4.1 Structure and Bonding in Alkenes

The filled  molecular orbital is the  bond EPM of Ethylene The filled  molecular orbital is the  bond Most of the important reactions of alkenes involve the electrons of the  bond 4.1 Structure and Bonding in Alkenes

Rules for Naming Alkenes Find the longest chain that includes the double bond and name this as the parent chain by changing the ending from “-ane” to “-ene” Hexane  Hexene If there are two possibilities, choose the principle chain to be the one with the greatest number of double bonds Number the chain so that the double bond has the lowest number possible and place that number before the parent chain The double bond takes precidence over the substituents when numbering the chain Identify the substituents with number and name

Problems Name the following:

If there are more than one double bonds in the parent chain, identify each double bond’s position by number and indicate how many are present using Greek prefixes preceded by the letter “a.” Example: -adiene, -atriene, etc.

Problems Name the following:

Substituents Containing Alkenes Some common groups with alkenes Nonsystematic traditional names 4.2 Nomenclature of Alkenes

Problems Name the following compounds:

Name the following compounds: Problems Name the following compounds: CH3CH=CHCH3

Stereoisomers: Molecules that have the same molecular formula, same atom connectivity, but a different spatial arrangement Cis-trans isomers: Recall: Constitutional/Structural Isomers: same molecular formula, different atom connectivity

Restricted Rotation of Alkenes 4.1 Structure and Bonding in Alkenes

Properties of 2-butene Cis-2-Butene Trans-2-Butene M.P. = -139°C B.P. = 3.7°C Density = 0.6213 g/mL Trans-2-Butene M.P. = - 105°C B.P. = 1°C Density = 0.6041 g/mL

Requirements for Cis-Trans Isomerism Only when both carbons are bonded to two different groups are cis-trans isomers possible Compounds that have one of their carbons bonded to two identical groups can’t exist as cis-trans isomers

Problems Which of the following compounds can exist as pairs of cis-trans isomers? Draw each cis-trans pair. CH3CH=CH2 CH3CH2CH=CHCH3 (CH3)2C=CHCH3 ClCH=CHCl There are two isomers for 3-methyl-2-pentene. Draw both of them. Which one is cis and which one is trans?

E,Z Nomenclature System cis vs trans is not always clear Cahn-Ingold-Prelog system: Assign priorities Z (zusammen): “together” On Zee Zame Zide E (entgegen): “across” E = Enemies = On Opposite Sides 4.2 Nomenclature of Alkenes

Priority Assignment Rules Examine each of the double bond carbons separately. Identify the two atoms directly attached to the C and rank them according to atomic #. Higher atomic # = higher priority Higher isotopic mass = higher priority

If the first atoms connected to the double bond carbon are the same, continue moving outward until the first point of difference

Multiple-bonded atoms are equivalent to the same number of single bonded atoms

Problems Using the Cahn-Ingold-Prelog rules, identify whether the following molecules are E or Z:

Problems Give the IUPAC names for each of the following molecules, including the E, Z designation.

What’s the Structure of C6H10? Saturated is C6H14 Therefore 4 H's are not present So, what does it look like? Double bond(s)? Triple bond(s)? Ring(s)? Ring and double bond?

Degree of Unsaturation Gives info on number of rings and/or p bonds Maximum # of H’s in a hydrocarbon: CnH2n+2 Each ring and/or p bond reduces number of hydrogens by 2 aka: unsaturation number

For C6H10 the unsaturation # is 2 Two double bond Two rings One double bond and a ring One triple bond

Halogens are counted as 1 H: Calculating Degree of Unsaturation for Compounds Containing Elements Other than Carbon and Hydrogen Halogens are counted as 1 H: 4.3 Unsaturation Number

Nitrogen increases H count by 1:

Ignore Oxygens O’s form two bonds, don’t affect the formula of an equivalent hydrocarbon No change in number of H atoms if insert an O into a hydrocarbon compound

Calculate the degree of unsaturation for the following molecules Problems Calculate the degree of unsaturation for the following molecules C12H20 C4H6 C6H5N C6H5NO2 C8H9Cl3 Draw as many structures as you can for #2

Alkene Stability Who’s more stable?

Product Ratio Interconversion between cis and trans isomers can be made to happen using a strong acid catalyst More stable product is favored

Heats of combustion ΔH°combustion = -2685.5 kJ/mol ΔH°combustion = -2682.2 kJ/mol The cis configuration is more strained = higher energy

Heats of Hydrogenation More stable alkene gives off less heat

Effects of Branching on Alkene Stability

Relative Stabilities of Alkene Isomers From heats of hydrogenation (DH°hydrog): More alkyl substituents on double bond → more stable alkene In general, the number of alkyl groups is more important than the identity

Hyperconjugation A stabilizing interaction between a vacant p orbital or anti-bonding π orbital on one atom with the electrons in a neighboring σ bond (usually C-H or C-C) In alkenes, occurs between the unfilled anti-bonding C=C π bond orbital and the electrons of a C-H σ bond of a neighboring substituent More substituents present = more opportunities for hyperconjugation = more stable alkene

Bond Strength sp2-sp3 bond is stronger than sp3-sp3 More highly substituted alkenes have higher ratio of sp2-sp3 to sp3-sp3

Addition Reactions of Alkenes Characteristic reaction of alkenes The p bond of carbon-carbon double bond is: Broken Nucleophilic Three main types of alkene addition rxns: Rxn with hydrogen halides (ex: HCl) Catalytic hydrogenation Hydration

Alkene Reactions with Hydrogen Halides Called “Electrophilic Addition of Alkenes” HF, HCl, HBr, HI add to alkenes → alkyl halides HF is rarely used due to its extreme toxicity Lung and cornea damage Eventual cardiac arrest and death

Electrophilic Addition Energy Path Two step process First transition state is high energy point

Problems Draw the mechanism for the following reactions

Writing Organic Reactions

Markovnikov’s Rule In the addition of HX to an alkene: the H attaches to the carbon with fewer alkyl substituents the carbocation forms on the more substituted carbon of the C=C the X attaches to the more substituted carbon When one of the possible isomer products predominates, the reaction is said to be regiospecific or regioselective

Problems Complete the following reaction showing the complete mechanism What alkene would you start with to prepare the following alkyl halide?

Regioselectivity of Addition Unsymmetrical alkenes can give regioisomers If both carbons of the C=C have similar substitution, then reaction is not regiospecific

Carbocation Structure and Stability The more highly substituted carbocation is more stable Formation of more substituted carbocation intermediate is favored

Why? Inductive Effect Electrons from groups neighboring the carbocation can shift toward the positive charge Larger groups can “donate” electrons more easily than H can

Hyperconjugation 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 Alkyl groups can share electrons with the empty neighboring p orbital

Problems Predict the products of the following reactions:

Problems Which alkenes would you start with to prepare the following alkyl halides?

Carbocation Rearrangements Sometimes, we get an unexpected product In a Carbocation Rearrangement, a group (alkyl, aryl, or hydride) moves to a different position so that a more stable carbocation can form

Alkyl Shift

Hydride Shift

Hydride shifts in biological molecules

Problem On treatment with HBr, vinylcyclohexane undergoes addition and rearrangement to yield the following product. Propose a mechanism to account for this result

Catalysis Catalyst: A substance that increases the reaction rate without being consumed Lowers the Ea Heterogeneous Homogeneous 4.9 Catalysis

Catalytic Hydrogenation of Alkenes Catalyst: Pt or Pd as In powdered form on carbon Heterogeneous catalyst

Aromatic  bonds are less reactive Example: Aromatic  bonds are less reactive 4.9 Catalysis

Problems What product would you obtain from the catalytic hydrogenation of (CH3)2C=CHCH2CH3? What product would you obtain from the reaction of 3,3-Dimethylcyclopentene with Palladium on Carbon in the presence of H2?

Hydration of Alkenes Alkene Acid-catalyzed addition of H2O across an double bond Homogeneous catalyst

Problems Draw out the mechanism and products for the following reactions:

Problems What alkenes might the following alcohols have been prepared from?

Nature produces catalysts called enzymes Enzymes Catalysis Nature produces catalysts called enzymes Most biological mechanisms would be too slow to be useful without enzymes 4.9 Catalysis

Principle of Microscopic Reversibility Generally, the reverse reaction follows the exact reverse of forward mechanism Dehydration is the reverse of hydration 4.9 Catalysis

Commonly, two or more reactions are in competition Reaction Rates Commonly, two or more reactions are in competition 4.8 Reaction Rates

Forward and reverse reaction have same transition state (‡) The Transition State Forward and reverse reaction have same transition state (‡) 4.8 Reaction Rates

Molecules must possess enough energy to get over the transition state The Energy Barrier Molecules must possess enough energy to get over the transition state Maxwell-Boltzmann distribution 4.8 Reaction Rates

Intermediates are formed in many reactions Multistep Reactions Intermediates are formed in many reactions Rate-determining step: Controls reaction rate 4.8 Reaction Rates

Hammond’s Postulate The structure and energy of the transition state can be approximated by the structure and energy of the intermediate 4.8 Reaction Rates