chapter 4 Flashcards
functional group
the atom or group in a molecule most responsible for the reaction the compound undergoes under a prescribed set of conditions
mechanism
how the structure of the reactant is transformed to that ofthe product
conversion of alcohol to alkyl halide
R-OH + H-Cl –> R-Cl + H-OH
R is alkyl group
Functional Groups
Class: alcohol
ROH
CH3CH2OH
Ethanol
Funtional Groups
Class: alkyl halide
RCl
CH3CH2Cl
Chloroethane
Carbonyl group
C=O
Functional class nomenclature CH3F
Methyl fluoride
Functional class nomenclature CH3CH2CH2CH2CH2Cl
Pentyl chloride
Functional class nomenclature
(CH3CH2)CHCH2CH2CH3
Br
1-Ethylbutyl bromide
Substitutive nomenclature (preferred method) CH3CH2CH2CH2CH2F
1-Fluoropentane
Substitutive nomenclature
CH3CHCH2CH2CH3
Br
2-Bromopentane
Substitutive nomenclature
CH3CH2CHCH2CH3
I
3-Iodopentane
Functional class Nomenclature of alcohol
- name the alkyl group attached to hydroxyl substituent (-OH)
- chain is numbered beginning at C to which -OH group is attached
- add the word alcohol
Substitutive Nomenclature of alcohol
- identify longest C chain that bears the -OH group
- replace -e ending with -ol
determining which direction chains are numbered
hydroxyl groups (-OH) outrank alkyl groups and halogen
Primary alcohols and alkyl halides
RCH2G
R is alkyl group
G is functional group
Secondary alcohol and alkyl halides
R2CHG
Tertiary alcohol and alkyl halide
R3CG
C-O and C-H bonds
are polar covalent
alcohols and alkyl halides
are polar molecules
boiling points of similar size molecules
are higher in polar molecules
bp of similar size molecules with hydrogen bonding
- these molecules have dipole-dipole attraction
- OH group of one molecule is hydrogen bond acceptor while -OH group of other molecule is hydrogen bond donor
in organic molecules
hydrogen bonding only occurs between OH or NH protons
alkyl halides
are insoluble in water
low-molecular weight alcohols
are soluble in water
-ex. methyl, ethyl, n-propyl, isopropyl
higher alcohols
become more hydrocarbon-like and less water soluble
hydrogen halides
order of reactivity parallels acidity
HI>HBr>HCl»HF
reactivity for alcohols
- tertiary alcohols are most reactive (converted to alkyl chlorides in minutes)
- primary alcohols are least reactive (conversion takes too long)
substitution
reaction of an alcohol with a hydrogen halide
mechanism
step-by-step pathway of bond cleavage and bond formation that leads from reactants to products
mechanism steps
- step 1: proton transfer (protonation=instantaneous) exothermic
- step 2: carbocation formation (dissociation=slow) endothermic
- step 3: reaction of cation with chloride ion (capture=fast) exothermic
Hammond’s Postulate
if two states are similar in energy, they are similar in structure
electrophilic
- electron loving
- electron pair acceptors
- react with Lewis bases
nucleophiles
- nucleus seekers
- react with electophiles
rate-determining step
- a reaction can proceed no faster than its slowest step
- in mechanism it’s often 2nd step (dissociation, making of carbocation)
Sn1 mechanism
- slowest step is considered to be unimolecular
- first order chemical reaction
unimolecular
only one species undergoes a chemical change
bimolecular
two molecules undergo a chemical change
stability of carbocation
- tertiary cation is most stable
- -because C-C bond is longer than C-H bond, allowing electrons to move easier
inductive effect
helps spread the + charge out on the C
-methyl groups (CH3) are better at donating electrons than H is
Sn2
methyl and primary alcohols
secondary order because step 2 is rate determining step
- if concentration of 1 compound is doubled, the rate of reaction doubles
- if concentration of both compounds is doubled, the rate of reaction is x4
2 ways of converting alcohol to alkyl halide
- add HCl to alcohol, cook for sometime, alkyl halide is produced
- add thionyl chloride (SOCl2)
Reaction rate
tertiary>secondary>primary
- governed by the activation energy of the slowest step, regardless of how many steps there are
- ex. SN2 reactions are still slower than SN1 even though there are fewer steps
Converting alcohol to alkyl halide with thionyl chloride
- requires use of pyridine in order to consume HCl that is produced
- used in primary and secondary alkyl chlorides
Halogenation of Alkanes
R-H +X2 –> R-X + H-X
CH4 + Cl2 –> CH3Cl + HCl
Alkanes to alkyl halides
- in order to make only CCl4 –> Cl2 has to be in excess at the beginning of the reaction
- in order to make only CH3Cl –> use excess CH4 and limit Cl2
Free radicals
- species that contain unpaired electrons
- neutral and nonpolar
homolytic cleavage
the bond between two atoms is broken so that each of them retains one of the electrons in the bond
heterolytic cleavage
one fragment of compound retains both electrons
chain-terminating steps
- the chain sequence is interrupted due to two odd-electron species combine to give an even-electron product
- when radicals combine and go away
- less likely to occur than propagation steps
bond breaking
- easier to break a secondary H-C bond than it is to break a primary H-C bond
- secondary radical is more stable than a primary radical
- just like tertiary/secondary carbocations are more stable than primary carbocations
halogenation of higher alkanes
- light is used to break Cl2 homolytically
- Cl atom is selective: it chooses secondary H 3.9 times more than primary H
relative rate (chloronation) vs relative rate (bromonation)
- chloronation: R3CH>R2CH2>RCH3 (5.2, 3.9, 1)
- bromonation: R3CH>R2CH2>RCH3 (1640, 82, 1)
bromine
-can do what chlorine does
-but bromine in more selective than chlorine
C-Br bond is longer than C-Cl so it cares more about which H to replace
-Br abstraction of H is more selective than Cl abstraction of H
stability of free radicals
methyl radical<tertiary radical
-tertiary radical is most stable
