6. alkenes: structure and reactivity · 13 6.4 cis-trans isomerism in alkenes carbon atoms in a...
TRANSCRIPT
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Based on McMurry’s Organic Chemistry, 7th edition
Dr Morteza Mehrdad
University of Guilan, Department of Chemistry,
Rasht, [email protected]
Alkenes: Structure and
Reactivityساختار و واکنش پذیری: آلکن ها
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Alkene - Hydrocarbon With
Carbon-Carbon Double Bond
Also called an olefin but alkene is better
Includes many naturally occurring materials
Flavors, fragrances, vitamins
طعم دهنده ها، خوشبوکننده ها، ویتامین ها
سقز از درخت بَنَه
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Why this Chapter?
C=C double bonds are present in most
organic and biological molecules
To examine consequences of alkene
stereoisomerism
To focus on general alkene reaction:
electrophilic addition
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6.1 Industrial Preparation and Use of
Alkenes
Ethylene and propylene are the most important organic
chemicals produced
Made by thermal cracking of light alkanes (petroleum)
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6.2 Calculating Degree of Unsaturation درجه سیرنشدگیمحاسبه
Relates molecular formula to possible structures
Degree of unsaturation درجه سیرنشدگی: number of multiple
bonds or rings
Formula for a saturated acyclic (سیرشده خطی) compound is CnH2n+2
Each ring or multiple bond replaces 2 H's
سیرنشده سیرشده
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Example: C6H10
Saturated is C6H14
Therefore 4 H's are not present
This has two degrees of unsaturation 2= سیرنشدگی درجه
Two double bonds?
or ring and double bond?
MW = 6 x 12 + 10 x 1= 82
or two rings?
or triple bond?
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Degree of Unsaturation With
Other Elements Organohalogens (X: F, Cl, Br, I)
Halogen replaces hydrogen
C4H6Br2 and C4H8 have one degree of unsaturation
Organoxygen compounds (C,H,O) - if connected by single bonds
These don't affect the total count of H's
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Organonitrogen compounds
Nitrogen has three bonds
So if it connects where H was, it adds a connection point
Subtract one H for equivalent degree of unsaturation in
hydrocarbon
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Method 1
Add the number of halogens to the number of hydrogens.
Ignore the number of oxygens.
Subtract the number of nitrogens from the number of hydrogens.
Method 2
Method 3
Summary - Degree of
Unsaturation
2
H-Hion Unsaturatof Degree
S)O, (ignore NX22CH
actsat
sat
سیرنشدگیدرجه =2 + تعداد اتم( ظرفیت-2)
2
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مثال: C20H32ClN
= 2
= 5
Diazepam, marketed as an antianxiety medication under the name
Valium, has three rings, eight double bonds, and the formula
C16H?ClN2O. How many hydrogens does diazepam have?
(Calculate the answer; don’t count hydrogens in the structure.)
= 2+16(4-2) +X(1-2)+1(1-2)+2(3-2)+1(2-2)
= 112
= -X+35
= 112
X= 13 C16H13ClN2O
سیرنشدگیدرجه =2 + تعداد اتم( ظرفیت-2)
2
2 + 20(4-2) + 32(1-2) +1(1-2)+1(3-2)
No.H + No.X = 32 + 1 = 33
- No.N = 33 -1 = 32
CnH2n+2 = C20H42
42-32 = 10
10 / 2 = 5
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Name the parent hydrocarbon—change ending to “–ene”
Number carbons in chain so that double bond carbons have lowest
possible numbers
Alkenes higher priority than alkanes: even shorter chain
Multiple “C=C“ are named as “diene” “triene” “tetraene” etc…
Rings have “cyclo” prefix—double bond always C#1, C#2
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6.3 Naming of Alkenes
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Many Alkenes Are Known by
Common Names
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6.4 Cis-Trans Isomerism in
Alkenes
Carbon atoms in a double bond are sp2-hybridized
Three equivalent orbitals at 120º separation in plane
Fourth orbital is atomic p orbital
Combination of electrons in two sp2 orbitals of two atoms
forms bond between them
Additive interaction of p orbitals creates a bonding orbital
Occupied orbital prevents rotation about -bond
Rotation prevented by bond - high barrier, about 268
kJ/mole in ethylene
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Rotation of Bond Is Prohibitive
This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).
Creates possible alternative structures
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The presence of a carbon-carbon double bond can create two possible structures
cis isomer - two similar groups on same side of the double bond
trans isomer - similar groups on opposite sides
Each carbon must have two different groups for these isomers to occur
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Cis, Trans Isomers Require That
End Groups Must Differ in Pairs
180°rotation superposes
Bottom pair cannot be superposed without breaking C=C
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6.5 Sequence Rules: The E,Z DesignationE , Z نامگذاریسیستم : قواعد ترتیب تقدم
Neither compound is clearly “cis” or “trans”
Substituents on C1 are different than those on C2
We need to define “similarity” in a precise way to
distinguish the two stereoisomers
Cis, trans nomenclature only works for disubstituted
double bonds
E/Z Nomenclature for 3 or 4 substituents
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E,Z Stereochemical Nomenclature
Priority rules
( قواعد ترتیب تقدم)
of Cahn, Ingold, and
Prelog
Compare where higher
priority groups are with
respect to bond and
designate as prefix
E -entgegen, opposite
sides
Z - zusammen,
together on the same
side
Hi(C1)-Low(C1)-Hi(C2)-Lo(C2)
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Ranking Priorities:
Cahn-Ingold-Prelog Rules
RULE 1
Must rank atoms that are connected at comparison point
Higher atomic number gets higher priority
Br > Cl > S > P > O > N > C > H
1 2
2 1
1
22
1
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RULE 2
If atomic numbers are the same, compare at next connection point at same distance
Compare until something has higher atomic number
Do not combine – always compare
Extended Comparison
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RULE 3
Substituent is drawn with connections shown and no double or
triple bonds
Dealing With
Multiple Bonds:
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6.6 Stability of Alkenesها پایداری آلکن
Cis alkenes are less stable than trans alkenes
Compare heat given off on hydrogenation: Ho
Less stable isomer is higher in energy
And gives off more heat
tetrasubstituted > trisubstituted > disubstituted > monosusbtituted
hyperconjugation ( مزدوج شدن موقت) stabilizes
ناپایدارتر از 2/8kJ/mol
ازدحام فضایی
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Comparing Stabilities of
Alkenes Evaluate heat given off when C=C is converted to C-C
More stable alkene gives off less heat
2/8 kJ/mol
(Gocis > Go
trans)
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trans-Butene generates 4 kJ less heat than cis-butene
120
116
CH3CH2CH2CH3
but the numbers aren’t exactly the same
4 kJ/mol (energy difference)
2.8 kJ/mole (quilibrium data)
for two reasons:
1. There is probably some experimental
error, since heats of hydrogenation
are difficult to measure accurately.
2. Heats of reaction and equilibrium
constants don’t measure exactly the
same thing.
Heats of reaction measure H°,
equilibrium constants measure G° a
(slight difference between the two)
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Alkenes become more stable with increasing substitution:
As a general rule, alkenes follow the stability order:
This order of stability is due to:•Hyperconjugation (مزدوج شدن موقت)
•bond strength ( قدرت پیوند)
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Hyperconjugation (مزدوج شدن موقت) Electrons in neighboring filled orbital stabilize vacant antibonding orbital net
positive interaction
برهم کنش پایدارکننده ای بین اوربیتال ضدپیوندی پرنشدهC=C و اوربیتال پربرروی استخالف های مجاورC-Hشده پیوند
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Alkyl groups are better than H
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Hyperconjugation involves a sigma orbital
component, usually a C-C or C-H bond.
Resonance involves pi orbitals
Alternative Explanation:
bond strength (قدرت پیوند)
sp3—sp2 bond > sp3—sp3 bond
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6.7 Electrophilic Addition of
Alkenes
General reaction
mechanism:
electrophilic addition
Attack of electrophile
(such as HBr) on
bond of alkene
Produces carbocation
and bromide ion
Carbocation is an
electrophile, reacting
with nucleophilic
bromide ion
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Two step process
First transition state is high energy point
Electrophilic Addition Energy
Path
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Electrophilic Addition for
preparations
The reaction is successful with HCl and with HI as well as
HBr
HI is generated from KI and phosphoric acid
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6.8 Orientation of Electrophilic Addition: Markovnikov’s Rule قاعده مارکونیکوف
In an unsymmetrical alkene, HX reagents can add in two different
ways, but one way may be preferred over the other
If one orientation predominates, the reaction is regiospecific
( جهت ویژه) Markovnikov observed in the 19th century that in the addition of HX
to alkene, the H attaches to the carbon with the most H’s and X
attaches to the other end (to the one with the most alkyl
substituents) This is Markovnikov’s rule.
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Addition of HCl to 2-methylpropene
Regiospecific – one product forms where two are possible
واکنشی که افزایش فقط در یکی از دوجهت ممکن صورت می گیرد–جهت ویژه If both ends have similar substitution,
Example of Markovnikov’s Rule
then not regiospecific
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Markovnikov’s Rule (restated) (به بیان دیگر) More highly substituted carbocation forms as intermediate
rather than less highly substituted one
Tertiary cations and associated transition states are more
stable than primary cations
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6.9 Carbocation Structure and
Stability Carbocations are planar and the tricoordinate carbon is
surrounded by only 6 electrons in sp2 orbitals
The fourth orbital on carbon is a vacant p-orbital
The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents (Hyperconjugation stabilizes C+)
خالی از الکترونpاوربیتال
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• The stability of carbocation (R+) can be determined by measuring energy needed to form it from R-X:
شکیل آن از الزم برای تانرژی گیری اندازه توسط کربوکاتیون می تواند تعیین پایداری •R-X آیدبدست
R-X R+ + :X-
• 3º alkyl halides dissociate to give R+ more easily than 2ºand 1º
تفکیک هالید نوع سوم بسیارسریع تر نوع اول و دوم است
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Inductive stabilization of cation species پایداری القایی
•Hyperconjugation
(موقتمزدوج شدن )آلکیل واقع بر روی تعداد گروههای هرچه
مزدوج رای کربوکاتیون بیشتر باشد، امکان بیشتری بد و ایجاد کربوکاتیونی پایدارتر، وجوشدن موقت
.خواهد داشت
ان کم الکترونی هرچه تعداد گروههای آلکیل بیشتری به کربن مثبت متصل باشد، چگالی الکترونی بیشتر و میزکاسته می شود( رنگ آبی) کربن مثبت
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6.10 The Hammond Postulateپذیره هاموند
If carbocation intermediate is more stable than another, why is
the reaction through the more stable one faster?
حصول چرا پایداری یک کربوکاتیون بر سرعت تشکیل آن اثر گذاشته و ساختار منهایی را تعیین می کند؟
The relative stability of the intermediate is related to an
equilibrium constant (Gº)
حدواسط کربوکاتیون به وسیله پایداری نسبی(Gº)تعیین می شود
The relative stability of the transition state (which describes
the size of the rate constant) is the activation energy (G‡)
به وسیله انرژی فعال سازی سرعت واکنش اما(G‡)بدست می آید
The transition state is transient and cannot be examined
ی گذرایی هستند و به دلیل عمر بسیار کوتاه خود، قابل بررسحاالت گذار، حاالتنیستند
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Transition State Structures
A transition state is the highest energy species in a reaction step
عبارت از پرانرژی ترین جزء در یک مرحله واکنش استحالت گذار یک By definition, its structure is not stable enough to exist for one vibration
قابل مشاهده باشندبرطبق تعریف، ساختار آنها به اندازه کافی پایدار نیست که But the structure controls the rate of reaction
اما ساختار آنها سرعت واکنش را کنترل می نمایند
So we need to be able to guess about its properties in an informed way
We classify them in general ways and look for trends in reactivity – the conclusions are in the Hammond Postulate
“The structure of the transition state resembles the structure of the nearest stable species. T.S. for Endergonic steps resemble products. T.S. for Exergonic steps resemble reactants.”
ساختار یک حالت گذار به ساختار نزدیک ترین گونه شیمیایی پایدار شباهت داردT.S. هافرآورده از لحاظ ساختاری، به انرژی خواه، مراحل ،
شباهت داردواکنش دهندگان ، از لحاظ ساختاری به انرژی زامراحل .T.Sو
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Examination of the Hammond
Postulate
A transition state
should be similar
to an intermediate
that is close in
energy
Sequential states
on a reaction path
that are close in
energy are likely
to be close in
structure - G. S.
Hammond
ط واکنش سریع تر حدواسندپایدارتری تولید می ک
واکنش آهسته تر منجر به تولید حدواسط پایدارتری
می شود
در مرحله انرژی خواه، انرژی حالت گذار ترازهای
و فرآورده، مشابه هستند
در مرحله انرژی زا، انرژی حالت گذارترازهای
و واکنش دهنده، مشابه هستند
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Competing Reactions and the
Hammond Postulate
Normal Expectation: Faster reaction gives stable intermediate
Intermediate resembles transition state حالت گذار حدواسط شبیه به است
یک تشکیل یک کربوکاتیون با پروتون دار شدنبنابراین حالت. آلکن، مرحله ای انرژی خواه است
گذار پروتون دار شدن آلکن، از لحاظ ساختاری، باشدباید به حدواسط کربوکاتیونی شباهت داشته
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6.11 Mechanism of Electrophilic Addition:
Rearrangements of Carbocations
Carbocations undergo structural rearrangements following set patterns
1,2-H and 1,2-alkyl shifts occur
Goes to give more stable carbocation
Can go through less stable ions as intermediates
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Alkyl Shifts Can Also Occur