Download - Ch. 4: Alcohols and Alkyl Halides
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Ch. 4: Alcohols and Alkyl Halides OHC XC
alcohol alkyl halide (X= F, Cl, Br, I)
4.1: Functional Groups - >11 million organic compounds which are classified into families according to structure and reactivity.
Functional Group (FG): a group of atoms, which are part of a larger molecule, that have characteristic chemical behavior. FG’s behave similarly in every molecule they are part of.
The chemistry of organic molecules is defined by the function groups it contains
H3C
HO
HH
H3CH
C CH
HH
H
H3C
HO
HH
H3CH
C CBr
HH
BrH H
Br Br
Br2
Br2
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C C
C C
C CC
CCC
H H
H
HH
Alkenes
Alkynes
Arenes
C C
Alkanes
Carbon - Carbon Multiple Bonds
C NO
nitro alkaneOC X
X= F, Cl, Br, IAlkyl Halide
Carbon-heteroatom single bonds
C O C CO
alcohols ethers
H
C N
amines
C SH C CS
thiols sulfides (thioethers)
acidic
basic
C CO
epoxide
C CSS
disulfide
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C H
O
C C
O
Carbonyl-oxygen double bonds (carbonyls)
aldehyde
ketones
C O
OH
C O
OC
carboxylic acid
ester
C Cl
O
acid chloride
C O
O
C
O
anhydrides
C N
O
amide
acidicCarbon-nitrogen multiple bonds
N
imine(Schiff base)
C C N
basic
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4.2: IUPAC Nomenclature of Alkyl Halides (please read) Use the systematic nomenclature of alkanes; treat the halogen as
a substituent of the alkane.
F- fluoro, Cl- chloro, Br- bromo, I- iodo
4.3: IUPAC Nomenclature of Alcohols 1. In general, alcohols are named in the same manner as
alkanes; replace the -ane suffix for alkanes with an -ol for alcohols
2. Number the carbon chain so that the hydroxyl group gets the lowest number
3. Number the substituents and write the name listing the
substituents in alphabetical order.
CH3CH2CH2CH3 CH3CH2CH2CH2OHOH
butane 1-butanol 2-butanol
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4.4: Classes of Alcohols and Alkyl Halides - Alcohols and alkyl halides are classified as according to the degree of substitution of the carbon bearing the halogen or -OH group
primary (1°) : one alkyl substituent secondary (2°) : two alkyl substituents tertiary (3°) : three alkyl substituents
Many alcohols are named using non-systematic nomenclature OH OH OHC
H3C
H3CH3C
benzyl alcohol(phenylmethanol)
allyl alcohol(2-propen-1-ol)
tert-butyl alcohol(2-methyl-2-propanol)
HOOH
ethylene glycol(1,2-ethanediol)
glycerol(1,2,3-propanetriol)
OHHOOH
OCH
HH H
OCR
HH HOC
R
HR HOC
R
RR H
1° carbon 2° carbon 3° carbon
methanol primary secondary tertiary
OH
2-methyl-2-pentanol
OH
3-phenyl-2-butanol
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4.5: Bonding in Alcohols and Alkyl Halides - the C-X bond of alkyl halides and C-OH bond of alcohols has a significant dipole moment.
HC O
HH
δ+Hδ+
δ-HC Cl
HH
δ+ δ-
µ = 1.9 µ = 1.7
4.6: Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces
H2O CH3CH2CH2CH3 CH3CH2CH2CH2Cl CH3CH2CH2CH2OH MW=18 MW=58 MW=92.5 MW=74 bp= 100° C bp= -0° C bp= 77° C bp= 116° C
CH3CH2Cl CH3CH2OH MW= 64.5 MW=60 bp= 12° C bp= 78° C
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Like water, alcohols can form hydrogen bonds: a non-covalent interaction between a hydrogen atom (δ+) involved in a polar covalent bond, with the lone pair of a heteroatom (usually O or N), which is also involved in a polar covalent bond (δ-)
O H
O Hδ- δ+
N H
N Hδ- δ+
C O C O
C Oδ-δ+
H HO
H
HO
H
HO
H
HO
H
HOH
HO
H
HO
H HO
H
HOH
HO
H
HO
H
HO
R
H OH
RO
R
H OH
RO
R
H O
RO
H
δ
δδ
δ
δ
δδ
δ
δ
δ
δ
δ
Hydrogen-bonds are broken when the alcohol reaches its bp, which requires additional energy
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4.7: Preparation of Alkyl Halides from Alcohols and Hydrogen Halides
R-OH + H-X R-X + HOH
Reactivity of the alcohol:
Reactivity of the H-X : parallels the acidity of HX HF < HCl < HBr < HI
CH
HR C
R
HR C
R
RROH OH OHC
H
HH OH << < <
increasing reactivity
Primary (1°) Secondary (2°) Tertiary (3°)Methyl
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Curved Arrow Convention
1. Curved arrows show the movement (flow) of electron during bond breaking and/or bond making processes. The foot of the arrow indicates where the electron or electron pair originates, the head of the arrow shows where the electron or electron pair ends up. .
A. The movement of a single electron is denoted by a curved single headed arrow (fishhook or hook).
B. The movement of an electron pair is denoted by a curved double headed arrow.
2. If an electron pair moves in on a new atom, another electron pair must leave so that the atom does not exceed a full valance of eight electrons. There are two common exceptions:
A. When an atom already has an incomplete valance (R3C+). B. With second row (or below) elements the octet rule may be violated.
3. The arrows completely dictate the Lewis structure of the product.
double-headed arrow single-headed
arrow
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Curved Arrow Convention
Other Suggestions for Proper Arrow Pushing: 4. The natural polarization of double bonds between unlike atoms is in the direction
of the more electronegative atom and this will be the important direction of electron movement.
5. In drawing a mechanism, the formal charges of atoms in the reactants may change in the product. Use your knowledge of Lewis structures and formal charge to determine this.
6. The first step in writing a mechanism is to identify the nucleophile (Lewis base) and the electrophile (Lewis acid). The first arrow is always from the nucleophile to the electrophile.
(CH3)3COH + HCl
The generally accepted mechanism for the reaction of t-butyl alcohol and HCl involves to give t-butyl chloride has three basic steps:
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Mechanism: (CH3)3COH + HCl (CH3)3CCl + H2O
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4.9: Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism
The rate of oxonium ion formation is very fast The rate of carbocation formation (dissociation of the oxonium ion) is slow The rate of reaction between the carbo- cation and for X- is fast
The overall rate is dependent of the slowest step (rate limiting step)
rate = k [oxonium ion] , where k is the rate constant
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4.10: Structure, Bonding, and Stability of Carbocations - Carbocations are sp2 hybridized and have a trigonal planar geometry
Substituents stabilize a carbocation through: a. Inductive Effects: shifting of electrons in a σ-bond in response to the electronegativity of a nearby atom (or group).
Carbon is a good electron donor. Substitution can also stabilize carbocations by donating electron density through the σ -bond.
3°: three alkyl groups donating electrons
2°: two alkyl groups donating electrons
1°: one alkyl group donating electrons
H HC
H
+H H
C
R
+R H
C
R
+R R
C
R
+
methyl: no alkyl groups donating electrons
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b. Hyperconjugation: The C-H σ-bond on the neighboring carbon lines up with the vacant p-orbital and can donate electron density to the carbon cation. This is a “bonding” interaction and is stabilizing. More substituted carbocations have more possible hyperconjugation interactions.
vacant p-orbital
4.11: Effect of Alcohol Structure on Reaction Rate. The order of reactivity for the reaction:
R3C-OH + H-X R3C-X + HOH where 3° alcohols are most reactive and 1° alcohols are least reactive, reflects the stability of the intermediate carbocation.
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The rate of a reaction is dependent of the activation energy (Eact) There is no formal relationship between ΔGact and ΔG° What is the structure of a transition state? How can the structures of the reactants and products affect ΔGact
The Hammond Postulate provides an intuitive relationship Between rate (ΔGact) and product stability (ΔG°).
Typical reaction coordinate less common
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The Hammond Postulate: The structure of the transition state more closely resembles the nearest stable species (i.e., the reactant, intermediate or product)
For an endothermic reaction (ΔG° > 0), the TS is nearer to the product. The structure of the TS more closely resembles that of the product. Therefore, factors that stabilize the product will also stabilize the TS leading to that product.
For an exothermic reaction (ΔG° < 0), the TS is nearer to the reactant. The structure of the TS more closely resembles that of the reactants.
ΔG°= 0 ΔG°> 0 ΔG°< 0
TS is halfway between reactant and products on the reaction coordinate
TS lies closer to the products than the reactants on the reaction coordinate
TS lies closer to the reactants than the products on the reaction coordinate
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Fig. 4.16
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4.12: Reaction of Primary Alcohols with Hydrogen Halides. The SN2 Mechanism:
Methyl and primary carbocations are the least stable, and they are not likely to be intermediates in reaction mechanism
RH2C-OH + H-X RH2C-X + HOH
SN2 (substitution-nucleophilic-bimolecular).
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4.13: Other Methods for Converting Alcohols to Alkyl Halides Preparation of alkyl chlorides by the treatment of alcohols with thionyl chloride (SOCl2)
R-OH + SOCl2 + base R-Cl + SO2 + HCl
Preparation of alkyl bromides by the treatment of alcohols with phosphorous tribromide (PBr3)
R-OH + PBr3 R-Br + P(OH)3
These methods work best on primary and secondary alcohols. They do not work at all for tertiary alcohols
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4.14: Halogenation of Alkanes
R-H + X2 R-X + H-X
Reactivity: F2 >> Cl2 > Br2 >> I2 4.15: Chlorination of Methane
Mechanism of free radical halogenation has three distinct steps 1. Initiation 2. Propagation 3. Termination
Free radical chlorination is not very useful for making alkyl chlorides: polychlorination, non-specific chlorination
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4.16: Structure and Stability of Free Radicals Free radical: species that contain unpaired electrons
O O N O Cl
Organic (alkyl) radicals are usually highly reactive. The stability and structure of alkyl radicals parallels those of
carbocations: HCH
HHCH
RRCH
RRCR
R< < <
methyl < primary (1°) < secondary (2°) < tertiary (3°)
Radicals are also stabilized by hyperconjugation
Increasing stability
HCH
HHCH
RRCH
RRCR
RH H H H
ΔH° (KJ/mol)= 435 410 397 380
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4.17: Mechanism of Chlorination of Methane Free-radical chain mechanism:
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4.18: Halogenation of Higher Alkanes – free radical chlorination of more substituted carbons is favored, reflecting the stability of the intermediate radical.
CCH
HCH
HCH
HH
H
HH CC
H
HCH
HCH
HH
H
HCl CC
H
ClCH
HCH
HH
H
HH+
(28 %) (72 %)
Cl2, hν
HCH
HCH
C
CH
HH
HH
H
ClCH
HCH
C
CH
HH
HH
H
HCH
HCCl
C
CH
HH
HH
H+
(63 %) (37 %)
four 2° hydrogenssix 1° hydrogens
one 3° hydrogensnine 1° hydrogens
Cl2, hν
HCH
HR
Primary (1°)hydrogens
1.0
HCR
HR
Secondary (2°)hydrogens
3.9
HCR
RR
Tertiary (3°)hydrogens
5.2
< <
Relative rates of free radical chlorination
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Free radical bromination is highly selective
HCH
HR
Primary (1°)hydrogens
1.0
HCR
HR
Secondary (2°)hydrogens
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HCR
RR
Tertiary (3°)hydrogens
1600
< <
HCH
HCH
C
CH
HH
HH
H
BrCH
HCH
C
CH
HH
HH
H
HCH
HCBr
C
CH
HH
HH
H+
Br2, hν
1 : 99
The propagation step for free radical bromination is endothermic, Whereas chlorination which is exothermic. According to the Hammond postulate the transition state for bromination should resemble the product radical, and therefore be more selective for the product going through the more stable radical intermediate