ORGANIC CHEMISTRY 1

Chapter 7 – instructor version

ALKENES and CYCLOALKENES

• Formulas & naming

• Bonding & molecular structure

• Physical & chemical properties

• Preparation & use

• Elimination rxn: E1 and E2

Based on Organic Chemistry, by L.G. Wade, 7th ed; Ch. 7Prepared by: Dr. Peter Ilich, St. John’s University

Queens, New York, Spring 2012

[7.1 & 7.4] Definition & naming:

- An alkEne has one or more pairs of doubly bonded carbon atoms, C=C, in its skeleton

formula: name:ethene

(ethylene)

propene

butene

2-butene

H2C=CH2

H2C=CH2-CH3

H2C=CH2-CH2-CH3

But – watch:

H3C-CH2=CH2-CH3

(or 1-butene)

[7.4] Alkene naming - practice

formula: name:

pentene (1-pentene)

2-pentene

3-heptene

H2C=CH2CH2CH2CH3

CH3-CH2-CH2-H2C=CH2-CH2-CH3

CH3CH2=CH2CH2CH3

C atom # 3, not # 4

C atom # 2

[7.4] Naming alkenes – more practice:

Formula, condensed: Name this compound:

5-methyl-2-hexene (“old” IUPAC)

(or, new IUPAC: 5-methylhex-2-ene)

2, not 4

5

Expand it: Or draw the C-skeleton:

Label the position of double bond (smallest number, 2), identify the substituent group and label its position (5):

[7.4] The rules for naming alkenes;- Longest chain – the amended rule

Name this alkene:

1st – expand it:

2nd - label the C-skeleton:

Not the longest chain, but the longest(and most branched) chain which contains all C=C

1 5

69

12

36

2-butyl-3-methylhex-1-ene

[7.4] Alkene naming continued: What do you name alkenes with 2 or more C=C?

1,3-pentadiene Name it:

Name this:1,3,7-octatriene

Note: 1,3-butadiene But not: 1,2-butadiene

Make a note: Allene Allene is not adiene; it is noteven an alkene

1,3-pentadiene

[7.4] Alkene naming – cycloalkenes

Formula 1:

Name 1:

Formula 2:

Name 2:

Make a note: Nodesignation ofalkene C-atoms as1º, 2º, 3º, & 4º,as in alkanes

cyclopropene

Radical group naming:

1,4-cycloheptadiene

methyl-methylene-

1-methylene-3-methylcyclopentane

2º C

not a 2º C

4º C

3º C

not a 3º C

not a 4º C

[7.4] Alkene naming – practice:

(A)

(B)

(C)

[7.2] Ethene electronic structure & geometry:

[7.2] El. structure & geo – comparison with alkanes

[7.7] Heats of hydrogenation, ΔHH2º

- Terminal vs. central C=C bond

3-methyl-1-butene

2-methyl-2-butene

-127

ΔHH2º [kJ mol-1]

-113

2-methylbutane

2-methylbutane

NOTE: The lower (more negative) the ΔHH2º number the less stable the alkene.

ΔHH2º of alkenes – what do they mean?

E

N

E

R

G

Y

ΔHfº H2(gas), C(soot) = 0 kJ mol-1 [Ch. 7, JBS 6th ed.]

-113 kJ -127 kJ

[ΔHfº (2-MeBu)= -182 kJ mol-1]

more stable

less stable

- Think of standard heats of formation, ΔHfº

[7.2] Bonding & molecular properties of alkenes

Alkene substitution & stability

LESS STABLE MORE STABLE MOST STABLE

And – the “take-home message” is:

The more substituted (or the fewer H atoms there are on) the C=C atoms the more stable the alkene.

[7.5] Alkenes – 2-D structure & isomerism

- The C=C part of an alkene is confined and locked into a plane:

You may not twist an alkene – like, for example, an ethane molecule, which twists ~ 1014 times

a second at room temperature – unless you “pay” 264 kJ/mol and break the π – bond.

ΔE = +264 kJ/mol

As a consequence, alkenes exist as 2-D isomers, as “E” (Germ., entgegen = oposite) and “Z” (Germ., zusamen = together) forms

C,C bond properties: alkenes vs. alkanes

(2Z)-butene

butane, gauche-

(2E)-butene

butane, anti-

[7.5] Alkenes – 2-D stereochemistry

- The cis-/trans-notation for 2-butene, CH3CH=CHCH3

cis-2-butene(methyls together)

trans-2-penteneor (2E)-pentene

trans-2-butene(methyls opposite)

2-D formula:

2-D formula: Practice:

[7.5] Alkene 2-D stereochemistry:

- The cis-/trans- notation for alkenes is being replaced by E(trans)-/Z(cis)- notation:

2nd: Identify and rank the groups at left; use the Sequence Rule

3rd: Do the same for the groups at right

5th: Name the compound:

1st: Expand the C=C fragment and dissect it vertically

4th: Compare the positions of the first-ranked groups: (Z)- for the same, (E)- for the opposite

(Z)-3-chloropent-2-ene

[7.5] Alkene 2-D stereo – practice

Formula: Partial name, specify E/Z:

5-methyl-(2 ,4 )-heptadiene

3-methyl-(3 )-hexene

(1 ,3 )-decadiene

5-methyl-(1 ,3 )-cycloheptadiene

(A) Biologically important E-/Z-stereomers:

Oleic acid C18:1(9c) & C18:1(9t)

C18:1(9t) – trans-oleic acid: industrially prepared form of oleic acid (through partial hydrogenation); cannot be metabolized and forms insoluble deposits in blood vessels:

C18:1(9c) – cis-oleic acid, the common form of oleic acid which can bemetabolized:

(B) Human/vertebrate visual pigment – retinal (retinimine)

The retinal C11 cis- to trans-isomerization is the physical-chemical basis of our ability to see light

[ Palczewski et al., Science, 289, Issue 5480, 739-745]

The experimentally determined structure of (Rhod)opsin

Impact of light on rhodopsin in our retinas inducesretinal-opsin to isomerize at C11 from cis- to trans-form:

Trans-retinal separates from the opsin protein andthis event triggers within 10-3 [s] a cascade of signals:

And – this ishow you see:

[7.8] Macroscopic physical properties of alkenes

- Lower alkenes (C2 to C5) are gases @ R.T.

- Boiling point increases with C- number

- Boiling point within the same size alkenes decreases with branching (same as alkanes)

- Less dense than water (Gulf oil slick!); mixes poorly or not at all with water

- More polarizable – C=C bond – than alkanes; induced dipole – induced dipole interactions: they become slightly dipolar in electric field

[7.9] Preparation of alkenes:

- By elimination reactions

- Examples:

- E1 elimination of alkyl alcohols

- E1 elimination of haloalkanes

- E2 elimination of haloalkanes

- Other elimination reactions

[7.9] Preparation of alkene by elimination;

E1 dehydration rxn of alcohols, recapitulation:

Carbocation rearrangement (if possible):

Acidification & loss of water; note Δ:

β-elimination; Zaytsev regioselectivity:

[4] Preparation of alkenes by E2 rxn:

E2 elimination of haloalkanes:

Substrate: usually hindered

Reagent: very strong, bulky base

Reaction flow: concerted

Transition state: concerted reaction

Stereochemistry: anti- and syn-periplanar

[4] Preparation of alkenes by E2 reactions

Example:

When nucleophile, Nu, in an SN2 reaction is also a strong Broensted base, it can lead to a concerted elimination, or the so-called E2 reaction, with the following mechanism:

Reaction type: ELIMINATION, ERXN rate = krate * [substrate]*[base] =2nd order, E2

E2 – continued:

Regiochemistry (where it occurs) of E2 reactions:

Zaytsev, E1 (E2)

Hoffmann, E2 less stable

more stable

Make a note - similarities & differences:

Elimination regioselectivity: Zaytsev Hoffmann

E2 stereoselectivity – the definition:

E2 substrate stereochemistry – another view:

E2 – the substrate & the product stereochemistry

meso-1,2-dibromo-1,2-diphenylethane

(E)-1-Bromo-1,2-diphenylethene

E2 – stereoselectivity (Important!):

Practice:

Good E2 geometry:

Bad E2 geometry:

E2 practice: complete the reaction & name the product:

Summary of Ch. 7 – what have we learned today?