Structure and Synthesis of Alkenes – Comprehensive Study Notes

Structure and Synthesis of Alkenes

Introduction to Alkenes

Alkenes are a fundamental class of hydrocarbons characterized by the presence of at least one carbon–carbon double bond. This double bond imparts unique chemical reactivity and physical properties to alkenes.

• Definition: Alkenes are hydrocarbons containing carbon–carbon double bonds.

• Alternative Name: Alkenes are also called olefins, meaning “oil-forming gas.”

• Functional Group: The carbon–carbon double bond is the functional group responsible for alkene reactivity.

Bonding and Structure in Alkenes

The bonding in alkenes involves both sigma and pi bonds, resulting from the hybridization of carbon atoms.

• Sigma Bonds: The sigma bonds around the double-bonded carbons are formed by sp2 hybridized orbitals.

• Geometry: The molecular geometry is trigonal planar with bond angles of approximately 120°.

• Pi Bond: The unhybridized p orbitals (one on each carbon) overlap to form the pi bond, which is above and below the plane of the molecule.

Bond Lengths: The C–C bond in ethylene (an alkene) is shorter (1.33 Å) than in ethane (an alkane, 1.54 Å) due to increased s character and pi overlap.

Pi Bonding in Ethylene

The pi bond in ethylene is formed by the side-to-side overlap of unhybridized p orbitals, requiring the molecule to be coplanar for effective overlap.

• Each carbon in ethylene has one unpaired electron in a p orbital.

• This overlap creates a region of electron density above and below the plane of the molecule.

Cis-Trans Isomerism and Interconversion

Alkenes can exhibit geometric (cis-trans) isomerism due to restricted rotation around the double bond.

• No Rotation: Rotation around the C=C bond is not possible without breaking the pi bond (requires 264 kJ/mol).

• Cis Isomer: Similar groups on the same side of the double bond.

• Trans Isomer: Similar groups on opposite sides of the double bond.

Elements and Degrees of Unsaturation

Unsaturation refers to structural features that reduce the number of hydrogens in a molecule compared to a saturated hydrocarbon.

• Definition: An element of unsaturation decreases the number of hydrogens by two.

• Index of Hydrogen Deficiency: Double bonds and rings are elements of unsaturation.

Calculating Unsaturations:

• Find the number of hydrogens for a saturated compound: $(2 \times C) + 2$

• Subtract the actual number of hydrogens and divide by 2: $\frac{\text{(Saturated H) – (Actual H)}}{2}$

• This method does not distinguish between rings and multiple bonds.

Heteroatoms:

• Halogens: Count as hydrogens.

• Oxygen: Ignore in the calculation.

• Nitrogen: Add the number of nitrogens to the saturated hydrogen count.

Example Calculations:

• For $C_5H_8$: $(2 \times 5) + 2 = 12$; $\frac{12-8}{2} = 2$ unsaturations.

• For $C_4H_7Br$: $(2 \times 4) + 2 = 10$; $\frac{10-8}{2} = 1$ unsaturation (Br counts as H).

• For $C_6H_7N$: $(2 \times 6) + 2 + 1 = 15$; $\frac{15-7}{2} = 4$ unsaturations.

IUPAC Nomenclature of Alkenes

Systematic naming of alkenes follows specific rules to ensure clarity and consistency.

• Find the longest chain containing the double bond.

• Change the ending -ane to -ene.

• Number the chain so the double bond has the lowest possible number.

• In rings, the double bond is assumed to be between carbons 1 and 2.

Multiple Double Bonds: Use prefixes di-, tri-, tetra- before -ene to indicate the number of double bonds (e.g., buta-1,3-diene).

Alkenes as Substituents

Alkenes can act as substituents in larger molecules, with specific names for these groups.

• Methylene group: =CH2 (methylidene group)

• Vinyl group: –CH=CH2 (ethenyl group)

Cis-Trans Isomers and Cyclic Compounds

Not all alkenes show cis-trans isomerism; it depends on the substituents attached to the double bond.

• Trans cycloalkenes are not stable unless the ring has at least eight carbons.

• All cycloalkenes are assumed to be cis unless named otherwise.

E-Z Nomenclature

For alkenes with more than two different substituents, the E-Z system is used based on the Cahn-Ingold-Prelog priority rules.

• Z (zusammen): High-priority groups on the same side.

• E (entgegen): High-priority groups on opposite sides.

• Priority is assigned by atomic number.

Example: E-1-bromo-1-chloropropene (Br and Cl are high-priority groups).

Stereochemistry in Dienes

When a molecule contains more than one double bond, the stereochemistry of each must be specified (e.g., (3Z,5E)-3-bromoocta-3,5-diene).

Commercial Uses of Alkenes

Alkenes such as ethylene and propylene are important industrial chemicals used to produce polymers and other compounds.

• Ethylene: Used to make polyethylene, ethylene glycol, ethanol, acetaldehyde, acetic acid, vinyl chloride.

• Propylene: Used to make polypropylene, propylene oxide, propylene glycol, isopropyl alcohol, acetone.

Addition Polymers

Alkenes can undergo polymerization to form addition polymers, which are important materials in industry.

• Polypropylene: From propylene monomer.

• Poly(vinyl chloride) (PVC): From vinyl chloride.

• PTFE (Teflon®): From tetrafluoroethylene.

Physical Properties of Alkenes

Alkenes have distinct physical properties compared to alkanes.

• Boiling Points: Generally low, increase with molecular mass.

• Branching: Branched alkenes have lower boiling points.

• Density: Less dense than water.

• Polarity: Slightly polar due to polarizable pi bond and possible dipole moments.

Polarity and Dipole Moments

Cis alkenes have greater dipole moments than trans alkenes, resulting in higher boiling points for cis isomers.

• Example: cis-but-2-ene ($\mu = 0.33$ D, bp 4°C) vs. trans-but-2-ene ($\mu = 0$, bp 1°C).

Physical Properties Table

The following table summarizes the physical properties of representative alkenes:

Name	Structure	Carbons	Boiling Point (°C)	Density (g/mL)
Ethene (ethylene)	CH2=CH2	2	-104	0.632
Propene (propylene)	CH2=CHCH3	3	-48	0.636
cis-but-2-ene	CH3CH=CHCH3	4	4	0.66
trans-but-2-ene	CH3CH=CHCH3	4	1	0.66
1-pentene	CH2=CHCH2CH2CH3	5	30	0.66
cis-2-pentene	CH3CH=CHCH2CH3	5	37	0.66
trans-2-pentene	CH3CH=CHCH2CH3	5	36	0.66
1-hexene	CH2=CH(CH2)4CH3	6	63	0.67
cis-3-hexene	CH3CH2CH=CHCH2CH3	6	68	0.67
trans-3-hexene	CH3CH2CH=CHCH2CH3	6	67	0.67
1-octene	CH2=CH(CH2)6CH3	8	121	0.71
1-decene	CH2=CH(CH2)8CH3	10	171	0.74

Heat of Hydrogenation and Alkene Stability

Hydrogenation reactions are used to assess the stability of alkenes. The more substituted the double bond, the lower its heat of hydrogenation, indicating greater stability.

• Equation: $\Delta H^\circ$ (heat of hydrogenation) decreases with increasing substitution.

• Example: But-1-ene: $\Delta H^\circ = -125.0$ kJ/mol; trans-but-2-ene: $\Delta H^\circ = -114.6$ kJ/mol.

Relative stabilities can be compared using energy diagrams and heats of hydrogenation.

Cycloalkenes and Cyclopropene

Cycloalkenes exhibit unique properties due to ring strain and the geometry of the double bond.

• Small rings (e.g., cyclopropene) have highly strained double bonds due to compressed bond angles (60° vs. 120°).

• Rings with five or more members can accommodate double bonds with less strain.

Stability of Cycloalkenes

• Cis isomers are more stable than trans in small cycloalkenes due to ring strain.

• Trans double bonds require rings with at least eight carbons for stability.

• For cyclodecene and larger rings, trans and cis double bonds have similar stabilities.

Bredt's Rule

Bredt's Rule states that a bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.

• Example: Norbornane (bridged) cannot have a double bond at the bridgehead; decalin (fused) can.

Solved Problem 1: Alkene Stability

Given several bicyclic compounds, determine which can stably accommodate a double bond at the bridgehead position. Only those with sufficiently large rings (eight or more carbons) can do so without violating Bredt's Rule.

Additional info: These notes cover the first half of Chapter 7, focusing on structure, nomenclature, physical properties, and stability of alkenes, including cycloalkenes and stereochemistry. Synthesis and reaction mechanisms are covered in subsequent sections.