Premed · Premed · Organic Chemistry 1
Lecture 12: Alkenes: Structure and Synthesis
Organic Chemistry I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and bonding of alkenes (sp2 hybridization, pi bonds)
- Name alkenes using IUPAC nomenclature including E/Z designation
- Explain the relative stability of alkenes based on degree of substitution
- Describe the synthesis of alkenes by elimination reactions (E1 and E2)
- Describe the synthesis of alkenes by dehydration of alcohols
- Predict the major product of alkene-forming reactions using Zaitsev's and Hofmann's rules
- Calculate the degree of unsaturation from molecular formulas
Lecture Content
I. Structure and Bonding of Alkenes
Alkenes, also called olefins, are hydrocarbons containing at least one carbon-carbon double bond. Acyclic alkenes with a single double bond follow the general formula CnH2n, and each double bond or ring contributes one degree of unsaturation.
Each carbon of the C=C double bond is sp2 hybridized, with three sp2 orbitals forming sigma bonds at 120-degree angles in a trigonal planar arrangement. The remaining unhybridized p orbital on each carbon overlaps laterally to form the pi bond, creating a region of electron density above and below the molecular plane. The C=C bond is shorter (1.34 angstroms versus 1.54 for C-C) and stronger (approximately 614 kJ/mol versus 350 for C-C) than a single bond, with the pi bond contributing roughly 264 kJ/mol.
A defining feature of alkenes is restricted rotation about the double bond. Rotating around the C=C would require breaking the pi bond, an energetically prohibitive process requiring approximately 264 kJ/mol. This restricted rotation gives rise to cis/trans (E/Z) geometric isomerism and makes double bonds fundamentally different from single bonds in their three-dimensional behavior.
II. Nomenclature of Alkenes
IUPAC naming of alkenes begins with finding the longest carbon chain that contains the double bond, which serves as the parent chain. The -ane suffix is replaced with -ene, and the chain is numbered to give the double bond the lowest possible locant, with the position indicated by the number of the lower-numbered carbon. Substituents are named and numbered as usual. Examples include 2-butene, 3-methyl-1-pentene, and cyclohexene.
When each carbon of the double bond bears two different groups, E/Z nomenclature specifies the stereochemistry. CIP priority rules rank the two groups on each carbon, and if the higher-priority groups are on the same side of the double bond, the configuration is Z (from zusammen, meaning together); if on opposite sides, the configuration is E (from entgegen, meaning opposite). The E/Z system is more general than the older cis/trans system, which works only for simple cases. Several common names remain in widespread use, including ethylene for ethene, propylene for propene, isobutylene for 2-methylpropene, and styrene for phenylethene.
III. Stability of Alkenes
Alkene stability increases with the degree of substitution, following the order tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted. This trend is measured experimentally by comparing heats of hydrogenation: a less negative heat of hydrogenation indicates a more stable starting alkene. For the C4 isomers, 1-butene (monosubstituted) releases -126 kJ/mol, cis-2-butene (disubstituted) releases -120 kJ/mol, and trans-2-butene (disubstituted) releases -115 kJ/mol upon hydrogenation.
Trans alkenes are generally more stable than their cis counterparts due to reduced steric strain between substituents on opposite sides. An exception occurs in small-ring cycloalkenes: trans cycloalkenes are too strained to exist for rings smaller than eight carbons. The stability trend is explained primarily by hyperconjugation, in which adjacent C-H sigma bonds donate electron density into the pi* antibonding orbital of the double bond. More alkyl substituents provide more opportunities for hyperconjugation and therefore greater stabilization.
<image>Panel A: An energy diagram showing the heats of hydrogenation for four C4 alkene isomers. Each alkene is drawn and placed at its relative energy level, with an arrow pointing down to butane (the common hydrogenation product). The least exothermic reaction (trans-2-butene) starts at the lowest energy (most stable alkene), while 1-butene is the highest energy (least stable). Numerical values of delta H are shown for each. Panel B: A diagram illustrating hyperconjugation in a substituted alkene: the filled C-H sigma orbital on an adjacent alkyl group overlaps with the empty pi* antibonding orbital of the C=C double bond, with orbital shapes drawn.</image>
IV. Synthesis of Alkenes: Elimination from Alkyl Halides
E2 elimination, reviewed from Lecture 10, employs a strong base to convert an alkyl halide into an alkene plus HX through a concerted, one-step mechanism requiring anti-periplanar geometry. With most bases, the Zaitsev product (more substituted alkene) predominates, while bulky bases like KOtBu give the Hofmann product (less substituted alkene).
E1 elimination operates under different conditions: weak bases, polar protic solvents, and tertiary or secondary substrates. The reaction proceeds through a carbocation intermediate and also follows Zaitsev's rule, but rearrangements are possible. Common bases for E2 reactions include NaOEt/EtOH (standard, gives Zaitsev product), KOtBu/tBuOH (bulky, gives Hofmann product), NaOH (effective with tertiary substrates), and DBU (a strong, non-nucleophilic base).
V. Synthesis of Alkenes: Dehydration of Alcohols
Acid-catalyzed dehydration converts alcohols to alkenes by removing water. The reaction requires a strong acid catalyst such as H2SO4 or H3PO4 along with heat. The mechanism depends on the alcohol class. Tertiary alcohols follow an E1 pathway: protonation of the hydroxyl group converts it to a good leaving group (water), which departs to form a carbocation, followed by loss of a beta-proton to generate the alkene. Secondary alcohols also follow E1 but require higher temperatures. Primary alcohols are dehydrated through an E2 mechanism at very high temperatures (approximately 180 degrees C with H2SO4), though they may also proceed through E1 with rearrangement.
Zaitsev's rule applies to dehydration: the more substituted alkene is the major product. The ease of dehydration follows the order tertiary > secondary > primary, reflecting carbocation stability for the E1 pathway. Carbocation rearrangements via 1,2-hydride and methyl shifts are common and can produce unexpected products, so one should always check for rearrangement possibilities.
VI. Synthesis of Alkenes: Other Methods
Several additional methods for alkene synthesis are available. The Hofmann elimination of quaternary ammonium salts (R4N+ treated with OH- and heat) gives the less substituted (Hofmann) alkene as the major product, which is useful when this regiochemistry is desired. The Cope elimination involves thermal decomposition of amine oxides through a syn elimination with mild conditions.
The Wittig reaction deserves special mention as a powerful method for converting a ketone or aldehyde directly into an alkene. It employs a phosphorus ylide (Wittig reagent) of the form R2C=PPh3, which reacts with a carbonyl compound (R2C=O) to produce the desired alkene (R2C=CR'2) along with triphenylphosphine oxide (O=PPh3) as a byproduct. The great advantage of the Wittig reaction is its ability to create specific alkenes with precisely defined positions of the double bond.
VII. Degree of Unsaturation (Index of Hydrogen Deficiency)
The degree of unsaturation is a powerful tool for deducing structural features from a molecular formula. It is calculated using the formula IHD = (2C + 2 + N - H - X) / 2, where C is the number of carbons, H the number of hydrogens, N the number of nitrogens, and X the number of halogens. Oxygen and sulfur do not affect the calculation.
Each degree of unsaturation corresponds to either one double bond (C=C, C=O, or C=N) or one ring. A triple bond accounts for two degrees of unsaturation, and a benzene ring accounts for four (three double bonds plus one ring). For example, C6H12 (IHD = 1) could be cyclohexane or a hexene. C6H6 (IHD = 4) immediately suggests benzene. C4H8O (IHD = 1) could contain a C=C, a C=O, or a ring.
<image>Panel A: Three example molecular formulas (C5H10, C5H8, C6H6) with their IHD calculations worked out step by step, and possible structural candidates drawn for each (cyclopentane/1-pentene for IHD=1, cyclopentene/1,3-pentadiene for IHD=2, benzene for IHD=4). Panel B: A summary showing how different structural features contribute to degrees of unsaturation: one ring = +1, one double bond = +1, one triple bond = +2, benzene ring = +4. Each feature is illustrated with a small structural diagram.</image>
VIII. Planning Alkene Synthesis: Retrosynthetic Analysis
Retrosynthetic analysis involves working backward from the desired alkene product to identify suitable starting materials and conditions. The process begins by identifying the target alkene, determining which elimination would produce it, and then identifying the alkyl halide or alcohol precursor.
Several key considerations guide the planning. The choice between Zaitsev and Hofmann bases determines the regiochemistry of the product. The stereochemistry depends on whether E2 (which gives anti-elimination products) is used and whether E/Z selectivity can be controlled. Conditions that produce carbocation intermediates (SN1/E1) risk unwanted rearrangements and should be avoided when regiochemical purity is important.
For example, to synthesize 1-butene from 2-bromobutane, the Hofmann (less substituted) product is needed, so a bulky base such as KOtBu in tert-butanol provides E2 conditions that favor 1-butene. To synthesize 2-methyl-2-butene from 2-methyl-2-butanol, the Zaitsev product is desired, so acid-catalyzed dehydration with H2SO4 and heat produces the trisubstituted alkene through an E1 mechanism.
