Alkenes - Synthesis and Reactions Alkenes and Alkynes Alkenes - Synthesis and Reactions Structure and Properties Nomenclature Synthesis of Alkenes Reactions of Alkenes

Structure and Properties Alkene: hydrocarbon with one or more C-C double bond also called olefin C=C consists of 1 s bond and 1 p bond Ethylene ethene

Structure and Properties C=C is a functional group BDE (s bond) = ~83 kcal/mol BDE (p bond) = ~ 63 kcal/mol p bond is weaker than s bond reactions take place at the p bond sp2 hybridization trigonal planar

Structure and Properties Trigonal planar geometry approximately 120o bond angle for alkenes vs. ~109.5o bond angle for alkanes Double bonds are shorter than single bonds.

Structure and Properties Alkanes saturated hydrocarbons each C has the maximum # of H’s possible Alkenes unsaturated hydrocarbons fewer H atoms per C than an alkane capable of adding hydrogen

Structure and Properties Element of unsaturation a structural feature that reduces the number of hydrogen atoms by 2 relative to the corresponding alkane ring p bond used to help determine possible structures

Structure and Properties Elements of unsaturation = 1/2 (2C + 2 - H) C6H12 EU = ½ (2x6 +2 – 12) = 1

Structure and Properties Example: Calculate the elements of unsaturation for C4H8. Draw 5 structural isomers with this formula.

Structure and Properties 5 structural isomers of C4H8

Structure and Properties To determine the elements of unsaturation for compounds with heteroatoms (atoms other than C and H): use same formula as given previously BUT Each halogen counts as a hydrogen atom Ignore any oxygen atoms Each nitrogen counts as 1/2 C

Structure and Properties Example: Calculate the elements of unsaturation for C6H9ClO. Draw at least 4 structural isomers.

Structure and Properties 4 possible structural isomers The structures you draw should contain reasonable functional groups….i.e. don’t make up strange functional groups!

IR Alkenes have two characteristic peaks in the IR: sp2 C-H at >3000 cm-1 C=C at ~1620 – 1680 cm-1 Conjugated alkene C=C is at lower frequency Isolated alkene C=C is at higher frequency C=C peak has variable intensity but is typically weak to moderate.

sp2 C-H Alkene C=C sp3 C-H

Nomenclature ethene propene ethylene propylene 2-methylpropene Alkenes can be named using either IUPAC names or common names. ethene ethylene propene propylene 2-methylpropene isobutylene Blue = IUPAC Red = common

Nomenclature Hexane hexene cyclopentane cyclopentene To name alkenes: Find the longest continuous chain (or ring) that contains the double bond. Base name = name of corresponding alkane or cycloalkane with ending changed to “ene” Hexane hexene cyclopentane cyclopentene

Nomenclature To name alkenes: Number from the end of the chain closest to the double bond the double bond is given the lower number of the two double-bonded carbons Cycloalkenes: double bond is always between carbons 1 and 2 5 1 2 6 4 4 2 1 5 3 3

Nomenclature a substituted a substituted 2-hexene cyclopentene Place the number of the double bond in front of the base name of the alkene (omit the number for cycloalkenes unless > 2 double bonds) a substituted 2-hexene a substituted cyclopentene a substituted hex-2-ene Newer IUPAC system places the position number just before the “ene” ending

trans-6-chloro-5-methyl-2-hexene trans-6-chloro-5-methylhex-2-ene Nomenclature Name substitutent groups in the same manner as in alkanes. trans-6-chloro-5-methyl-2-hexene or trans-6-chloro-5-methylhex-2-ene 3-bromo-4-methylcyclopentene

Nomenclature Alkenes as substitutents (often named using common names) Methylene group 3-methylenecyclohexene vinyl group 3-vinyl-1,5-hexadiene 3-vinylhexa-1,5-diene Allyl group Allyl chloride

Nomenclature For compounds that show geometric isomerism, add the appropriate prefix: cis trans OR E Z NOTE: Cycloalkenes are assumed to be cis unless otherwise indicated.

Nomenclature Cis/trans isomers Cis: 2 identical groups located on the same side of the double bond Trans: 2 identical groups located on opposite sides of the double bond

Nomenclature Example: Name the following compounds.

Nomenclature Some compounds form geometric isomers that cannot be named using the cis/trans nomenclature Cis/trans nomenclature can’t be used: two identical groups are not attached to adjacent carbons in the C=C

Nomenclature The E-Z system of nomenclature for geometric isomers: Break the double bond into two halves Separately, assign priorities to the groups on each carbon in the double bond using the Cahn-Ingold-Prelog rules (R & S configuration rules) 1 1 2 2

Nomenclature (Z)-1-bromo-1-chloropropene Z (Zusammen) isomer both high priority groups are on the same side of the double bond similar to cis E (Entgegen) isomer high priority groups are on the opposite side of the double bond similar to trans (Z)-1-bromo-1-chloropropene

a substituted octatriene Nomenclature Naming alkenes with more than one double bond: Make sure that the longest chain includes as many C=C as possible. 2 C=C diene 3 C=C triene 4 C=C tetraene a substituted octatriene

Nomenclature 3-bromo-2, 4, 6-octatriene 3-bromoocta-2,4,6-triene Show the location of each double bond Designate the isomer present for each double bond (use location and E or Z) 3-bromo-2, 4, 6-octatriene 3-bromoocta-2,4,6-triene (2Z,4E,6E)-3-bromo-2,4,6-octatriene (2Z,4E,6E)-3-bromoocta-2,4,6-triene

Nomenclature Example: Name the following compounds.

Nomenclature Example: Draw the following compounds. cis-3-methyl-2-pentene 1-ethylcyclohexene (2E, 4Z)-2,4-hexadiene Remember: You must show the trigonal planar geometry around the C=C.

Uses and Physical Properties Alkenes are important intermediates in the synthesis of polymers, drugs, pesticides, and other chemicals. Ethylene is used as a feedstock for: ethanol ethylene glycol (antifreeze) acetic acid Propylene is used as a feedstock for: isopropyl alcohol acetone

Uses and Physical Properties Alkenes are important “monomers” for the production of polymers like poly(vinyl chloride), and Teflon.

Uses and Physical Properties Similar to alkanes Density ~0.6 g/mL to ~ 0.7 g/mL Boiling Point increases with increasing MW decreases with branching Polarity relatively non-polar insoluble in water

Stability of Alkenes The heat of hydrogenation is used to compare the relative stabilities of alkenes. Heat of hydrogenation: The heat released (DH) during a catalytic hydrogenation Catalytic hydrogenation: the addition of H2 to a double (or triple) bond in the presence of a catalyst Pt

Stability of Alkenes As the heat of hydrogenation becomes more negative, the stability of the alkene decreases.

Stability of Alkenes More highly substituted double bonds are more stable larger angular separation between the bulky alkyl groups

Stability of Alkenes For acyclic alkenes, trans isomers are more stable than cis isomers. Trans isomers of cycloalkenes with fewer than 8 carbons are unstable. Large amount of ring strain Because of ring strain, cycloalkenes with less than 5 carbons in the ring are less stable than those with 5 or more carbons.

Stability of Alkenes Example: Which of the following alkenes is more stable.