Chem 341 • Organic Chemistry I Lecture Summary 16 • October 01, 2007

Home , Pi bond, Secondary (chemistry), Tertiary (chemistry)

Chapter 6 - Alkenes: Structure and Reactivity Nomenclature Double bond geometry is important in biology. For example, the trans to cis isomerization of retinal when struck by a photon is the signaling trigger for the brain’s recognition of light. Thus, a double bond stereochemistry change is critical for vision.

β-carotene from carrots

metabolized When trans-retinal absorbs a photon of light, it isomerizes one of the double bonds to the cis isomer. This changes the shape of the molecule, OH and that changes the shape of the rhodopsin protein that it is buried in. This sends a signal to vitamin A the brain that the eye has been struck by a photon. This is critical for vision. metabolized

hν O trans-retinal

cis-retinal O

Double bonds have restricted rotation due to the side-to-side overlap of the orbitals to create the pi-bond. Thus, if it were to rotate, the pi-bond would be broken. Similar to the rigidity of cycloalkenes, the rigid nature of a double bond creates cis/trans stereoisomers. This must be added to the front of the name.

rotate

trans-2-butene cis-2-butene pi-bond broken Alkenes with four different groups attached to it also have different stereoisomers. However, our cis-trans designation is not adequate for describing their difference because one wouldn’t know ©2007 Gregory R. Cook page 1 Chem 341 North Dakota State University which groups you were referring to as being on the same side or opposite sides. Thus, we have another designation that describes the relative positions of the groups on each end of the double bond that have the highest priority (see below). We identify the highest priority group on either end of the double bond and if they are on the same side of the alkene we designate it as a Z isomer (zusammen - “together”). If they are on opposite sides, we use the E designation (entgegen - “opposite”). Cahn-Ingold-Prelog rules for assigning priorities. 1) Look at the atom directly connected to the carbon of the double bond. Rank the atoms according to their atomic number. The higher atomic number gets priority over the lower. 2) If the distinction cannot be made after the first atom, look at the next level of atoms attached to thos groups. Only look out as far as you need to make a distinction and NO FURTHER.

H C H Cl CH3 Cl has a higher CH2CH3 has a higher H priority than H. C C priority thaN CH3. H Here we can H CH CH 2 3 C H make a distinction C between the two An E alkene groups 3) Multiple bonds are equivalent to the same number of single bonds IN BOTH DIRECTIONS. It is useful to draw out a group with multiple bonds adding in the imaginary groups.

H C H Here we can make a distinction CH2 CH C H C C between the two Cl CH CH2 CH CH2 groups H C C imagine on the left H CH CH carbon, the double 3 H bond was attached to a H CH 3 CH2 and on the right C H A Z alkene carbon it was attached C H to a CH C H H H Cis alkenes are less stable than trans alkenes. This is due to greater steric strain when putting substituents close to each other. For example, cis-2-butene is higher in energy by about 2.8 kJ/ mol than the trans isomer.

H H+ cat. H H H H H C C heat C H trans isomer is H H less crowded C C H C C H H H H C H H 24% 76%

©2007 Gregory R. Cook page 2 Chem 341 North Dakota State University We see that cis-alkenes are less stable than trans-alkenes due to steric crowding. This can be directly measured by measuring the heat of combustion or the heat of hydrogenation. These are useful techniques to compare the relative energies of molecules. Compare the heats of hydrogenation below. This is roughly equivalent to the difference in Gibbs Free Energy.

H2/Cat. H2/Cat.

ΔH° trans = -115 kJ/mol ΔH° cis = -120 kJ/mol

E cis-2-butene ΔG° cis

trans-2-butene ΔG° trans

butane reaction progress

Alkenes that have more alkyl substituents also show greater stability of the pi-bond. Compare the heats of hydrogenation below for a series of substituted alkenes. The increase in stability (lowering of energy) with more alkyl groups is due to greater hyperconjugation from adjacent alkyl-H sigma bonds.

ΔH° (hydrogenation) ΔH° (hydrogenation) -137 kJ/mol

-126 kJ/mol -113 kJ/mol

-126 kJ/mol -111 kJ/mol -115 kJ/mol

©2007 Gregory R. Cook page 3 Chem 341 North Dakota State University Hyperconjugation Molecular orbital theory says that a pi-bond consists of a filled (lower energy) bonding orbital and an unfilled (higher energy) antibonding orbital. Alkenes are stabilized by adjacent electron density from attached alkyl groups donating into this unfilled antibonding orbital.

antibonding p orbital p orbital

bonding

H H H C C H C

H H Carbocation Stability Hyperconjugation of adjacent sigma bonds also helps to stabilize a carbocation because the electron density of the bond is donating into the empty orbital. Thus, similar to alkenes, carbocations are more stable with more alkyl groups attached. A tertiary carbocation is more stable than a secondary carbocation which is more stable than a primary carbocation.

H H H C R C R > R C H > R C H R R H C

3° carbocation 2° carbocation 1° carbocation H H

Quiz of the day (from 10/1)