Transcript alkene structure, naming, stereochemistry & preparation

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. 7
Prepared 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:
H2C=CH2
H2C=CH2-CH3
H2C=CH2-CH2-CH3
ethene
(ethylene)
propene
butene
(or 1-butene)
But – watch:
H3C-CH2=CH2-CH3
2-butene
[7.4] Alkene naming - practice
formula:
name:
H2C=CH2CH2CH2CH3
CH3CH2=CH2CH2CH3
pentene (1-pentene)
2-pentene
C atom # 2
CH3-CH2-CH2-H2C=CH2-CH2-CH3
3-heptene
C atom # 3, not # 4
[7.4] Naming alkenes – more practice:
Formula, condensed:
Expand it:
Name this compound:
Or draw the C-skeleton:
2, not 4
5
Label the position of double bond (smallest number, 2),
identify the substituent group and label its position (5):
5-methyl-2-hexene (“old” IUPAC)
(or, new IUPAC: 5-methylhex-2-ene)
[7.4] The rules for naming alkenes;
- Longest chain – the amended rule
Name this alkene:
1st – expand it:
Not the longest chain, but the longest
(and most branched) chain which contains all C=C
1
2
2nd - label the C-skeleton:
5
1
6
9
6
3
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 this:
Note: 1,3-butadiene
Make a note: Allene
Name it:
1,3-pentadiene
1,3,7-octatriene
But not: 1,2-butadiene
Allene is not a
diene; it is not
even an alkene
[7.4] Alkene naming – cycloalkenes
Formula 1:
Formula 2:
Name 1: cyclopropene
Name 2: 1,4-cycloheptadiene
methyleneRadical group naming:
methyl-
1-methylene-3-methylcyclopentane
Make a note: No
designation of
alkene C-atoms as
1º, 2º, 3º, & 4º,
as in alkanes
3º C
not a 2º C
2º C
not a 4º C
not a 3º C
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
ΔHH2º [kJ mol-1]
-113
3-methyl-1-butene
2-methylbutane
-127
2-methyl-2-butene
2-methylbutane
NOTE: The lower (more negative) the ΔHH2º
number the less stable the alkene.
ΔHH2º of alkenes – what do they mean?
- Think of standard heats of formation, ΔHfº
ΔHfº
H2(gas), C(soot) = 0 kJ mol-1 [Ch. 7, JBS 6th ed.]
E
N
less stable
more stable
E
R
G
-113 kJ
-127 kJ
Y
[ΔHfº (2-MeBu)= -182 kJ mol-1]
[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:
ΔE = +264 kJ/mol
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.
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
2-D formula:
trans-2-butene
(methyls opposite)
cis-2-butene
(methyls together)
2-D formula:
Practice:
trans-2-pentene
or (2E)-pentene
[7.5] Alkene 2-D stereochemistry:
- The cis-/trans- notation for alkenes is being
replaced by E(trans)-/Z(cis)- notation:
1st: Expand the C=C fragment and dissect it vertically
2nd: Identify and
rank the
groups at
left; use the
Sequence
Rule
5th: Name the
compound:
3rd: Do the same for
the groups at right
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:
3-methyl-(3 )-hexene
5-methyl-(2 ,4 )-heptadiene
5-methyl-(1 ,3 )-cycloheptadiene
(1 ,3 )-decadiene
(A) Biologically important E-/Z-stereomers:
Oleic acid C18:1(9c) & C18:1(9t)
C18:1(9c) – cis-oleic acid, the common form
of oleic acid which can be
metabolized:
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:
(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
The experimentally determined structure of (Rhod)opsin
[ Palczewski et al., Science,
289, Issue 5480, 739-745]
Impact of light on rhodopsin in our retinas induces
retinal-opsin to isomerize at C11 from cis- to trans-form:
Trans-retinal separates from the opsin protein and
this event triggers within 10-3 [s] a cascade of signals:
And – this is
how 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:
Acidification & loss of water; note Δ:
Carbocation rearrangement (if possible):
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:
E2 – continued:
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, E
RXN rate = krate * [substrate]*[base] =2nd order, E2
Regiochemistry (where it occurs) of E2 reactions:
Zaytsev, E1 (E2)
Hoffmann, E2
Make a note - similarities & differences:
more stable
less stable
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?