Prelim Exam 1: Review of Reactions; Chemistry of Benzene; Alcohols and Phenols; Ethers (Chem 322 - Organic Chemistry) Flashcards
aromaticity
stability associated with benzene and related compounds that contain a cyclic conjugated system of 4n + 2 pi electrons
electrophilic aromatic substitution
an electrophile (E+) reacts with an aromatic ring and substitutes for one of the hydrogens
halogenation
substitution of a hydrogen on an aromatic ring with a halogen (-Cl, -Br, -I); occurs with the use of a catalyst or oxidizing agent
nitration
substitution of a hydrogen on an aromatic ring with a nitro group (-NO2); added with H2SO4; nitro-substituted product can be reduced to yield an arylamine (ArNH2)
sulfonation
substitution of a hydrogen on an aromatic ring with a sulfonic acid group (-SO3H); adding with fuming sulfuric acid (mixture of H2SO4 and SO3); is readily reversible and is favored in strong acid
hydroxylation
substitution of a hydrogen on an aromatic ring with a hydroxyl group (-OH); added with catalyst p-hydroxyphenylacetate-3-hydroxylase, molecular oxygen and coenzyme reduced flavin adenine dinucleotide (FADH2)
alkylation
substitution of a hydrogen on an aromatic ring with an alkyl group (-R); introduction of an alkyl group onto the benzene ring
acylation
substitution of a hydrogen on an aromatic ring with an acyl group (-COR)
Friedel-Crafts Reaction
alkylation reaction is carried out by treating an aromatic compound with an alkyl chloride (RCl) in presence of AlCl3 to generate a carbocation electrophile (R+) that loses a H+ to yield a substituted arene
limitations of Friedel-Crafts Alkylation
- Only alkyl halides can be used… 2. not functional when the aromatic ring is substituted by a strong electron-withdrawing group (such as carbonyl) or by a basic amino group that can be protonated… 3. difficult to stop reaction after a single substitution (polyalkylation is favored)… 4. skeletal rearrangement of alkyl carbocation electrophile sometimes occurs during reaction (especially when using a primary alkyl halide)
substituent effects on electrophilic aromatic substitution
affect reactivity, affect orientation of reaction
activating groups
donate electrons to the ring, making the ring more electron-rich, stabilizing the carbocation intermediate, and lowering the activation energy for its formation
deactivating groups
withdraw electrons from ring, making the ring more electron-poor, destabilizing the carbocation intermediate, and raising the activation energy for its formation
inductive effect
withdrawal/donation of electrons through sigma bond due to electronegativity (withdrawal of electrons by halogens, hydroxyl groups, carbonyl groups, cyano groups, and nitro groups) (donation of electrons by alkyl groups)
resonance effect
withdrawal/donation of electrons through a pi bond due to overlap of a p orbital on substituent with a p orbital on aromatic ring (withdrawal of electrons by carbonyl groups, cyano groups, and nitro groups) (donation of electrons by halogens, hydroxyl groups, and alkoxyl -OR groups)
orientation of EAS reaction
nature of substituent initially present on benzene ring determines position of second substituent
(Substituent Effects in EAS) CH3
activating; ortho/para-directing; weak donating inductive effect; no resonance effect
(Substituent Effects in EAS) OH, NH2
activating; ortho/para-directing; weak withdrawing inductive effect; strong donating resonance effect
(Substituent Effects in EAS) F, Cl, Br, I
deactivating; ortho/para-directing; strong withdrawing inductive effect; weak donating resonance effect
(Substituent Effects in EAS) NO2, CN, CHO, CO2R, COR, CO2H
deactivating; meta-directing; strong withdrawing inductive effect; strong withdrawing resonance effect
electron donating group
lone pairs of electrons on first atom
electron withdrawing group
first atom is multiple bonded to a more electronegative atom
electron withdrawing groups on aromatic rings
strong deactivators and meta directing: electron withdrawing inductive effect and electron withdrawing resonance effect
electron donating groups (except halogens) on aromatic rings
strong activators and ortho/para directing: electron donating inductive effect and electron donating resonance effect
halogens on aromatic rings
weak deactivators and ortho/para directing: electron withdrawing inductive effect and electron donating resonance effect
if directing effects of two groups agree with each other
a singular aromatic substitution product is formed because further substitution is directed to same position
if directing effects of two groups oppose each other
a mixture of aromatic substitution products is formed, with the major product having further substitution at the point where the more powerful activating group directs it
further substitution between two groups in a meta-disubstituted compound
rarely occurs because of hindrance at the middle position
deactivating substituents on aromatic rings
prevent Friedel-Crafts alkylation/acylation
ortho-position is not favored
if ortho/para directing substituent is bulky (ex: NHOCH3, t-butyl group)
reactivity / influence on EAS of substituents on aromatic ring
strong deactivators (electron-withdrawing groups) < weak deactivators (halogens) < weak activators (aromatic ring / alkyl groups) < strong activators (electron donating groups except halogens)
nucleophile aromatic substitution (NAS)
a nucleophile (Nuc:-) reacts with aromatic ring and substitutes for an attached halide/leaving group; occurs only if aromatic ring has an electron-withdrawing substituent in ortho/para position to the leaving group (to stabilize the anion intermediate through resonance) and is thus favored by electron-withdrawing substituents (which activate the rings in this process, compared to electron-donating groups that activate in EAS)
electron-withdrawing groups in NAS
ortho/para directing
benzyne
a highly reactive intermediate in some nucleophilic aromatic substitutions that has a benzene ring with a formal triple bond (from removal of two hydrogens); the triple bond uses sp^2 hybridized carbons instead of sp; there is one pi bond formed from p-p overlap and one pi bond formed from sp^2-sp^2 overlap
oxidation of alkyl side chains on aromatic compounds
uses KMnO4, H2O… involves reaction of CH bonds at position next to aromatic ring (can only occur if benzylic hydrogens are present)
bromination of alkylbenzene side chains
uses NBS… only occurs at benzyllic position (no mixture of products)
selective reduction of alkene on side chain of aromatic compound
uses H2, Pd/C… reduces alkene only
hydrogenation of aromatic ring
uses Pt/C or Rh/C (most effective)… reduces aromatic compound to cyclic compound with no multiple bonds
reduction of aryl alkyl ketones
uses H2, Pd/C (after original Friedel-Crafts acylation reaction)… converts any alkyl ketone prepared by Friedel-Crafts acylation into an alkylbenzene, avoiding carbocation rearrangement issues from using a primary alkyl halide; not compatible with presence of NO2 because NO2 is reduced to NH2 under these reaction conditions
limitations of Friedel-Crafts Reactions
cannot occur on strongly deactivated rings; will not work is amine is present; polyalkylation will occur without use of excess benzene; rearrangement of alkyl groups occurs
Br2/FeBr3
brominating agent that introduces a bromine onto an aromatic ring
Cl2/FeCl3
chlorinating agent that introduces a chlorine onto an aromatic ring
I2/CuCl2
iodizing agent that introduces an iodine onto an aromatic ring
HNO3/H2SO4
nitrating agent that introduces a nitro group onto an aromatic ring
SO3/H2SO4
sulfonating agent that introduces a sulfonic acid onto an aromatic ring
RCl/AlCl3
alkylating agent that introduces an alkyl group onto an aromatic ring
RCOCl/AlCl3
acylating agent that introduces an acyl group onto an aromatic ring
- Fe, H3O+ ; 2. HO-
reducing agent that reduces a nitro group on a benzene to an amine group
NaOH, H2O
substitutes an aryl halide for an hydroxyl group on an activated aromatic ring
NaNH2, NH3
substitutes an aryl halide for an amine group (through a benzyne intermediate) on an unactivated aryl halide
KMnO4 (H2O) (hot, concentrated)
oxidizing agent that converts alkylbenzenes to benzoic acid derivatives (converts an alkyl group to a carboxylic acid)
NBS
brominates an alkylbenzene side chain (only if a benzylic hydrogen is present)
H2, Rh/C
catalytically hydrogenates a benzene ring to its saturated cyclohexane
H2, Pd/C
reducing agent that reduces an aryl alkyl ketone to a alkyl group and reduces a nitro group to an amine group
Zn(Hg)/HCl
reducing agent that converts a carbonyl group of an aldehyde/ketone to a methylene (CH2) group
where a substituent directs further substitution is dependent upon/due to
charged positions on resonance structures of benzene; due to stability of carbocation intermediates
H3O+
removes sulfonic acid groups from aromatic ring
SO3H
used as a blocking substituent when trying to direct further substitution to a specific position (then later removed)
Mg
addition to a benzene with a halide yields a Grignard reagent (which is a good nucleophile)
- NaOH; 2. H+
converts a sulfonic acid group on a benzene to an hydroxyl group
bulky substituent
when substituent is branched at first point of attachment to aromatic ring
add blocking group
when only ONE product is desired (and not wanting to separate out mixtures)
alcohol
compounds that have OH bonded to a saturated, sp^3 hybridized carbon atom
enol
compounds that have OH bonded to a vinylic, sp^2 hybridized carbon
phenol
OH bonded to a carbon involved in an aromatic benzene ring
primary alcohol
an alcohol in which the hydroxyl (-OH) group is attached to a carbon that is attached to no more than one other carbon.
secondary alcohol
An alcohol in which the hydroxyl (-OH) group is attached a carbon that is attached to two other carbons.
tertiary alcohol
An alcohol in which the hydroxyl (-OH) group is attached to a carbon that is in turn attached to three other carbons.
alcohol nomenclature
- select longest carbon chain containing hydroxyl group and derive parent name by replacing -e ending with -ol… 2. number chain beginning at end nearest to hydroxyl group… 3. number substituents according to position on chain and write the name, listing substituents in alphabetical order and identifying position to which OH is bonded
phenol nomenclature
“phenol” as parent name and applicable numbering of substituents when present
properties of alcohols and phenols
higher boiling points due to hydrogen bonding; weakly basic and weakly acidic (as weak bases, they are reversibly protonated by strong acids to yield oxonium ion ROH2+ / as weak acids, they dissociate slightly in dilute aqueous solution by donating a proton to water to yield H3O+ and alkoxide ion RO- or phenoxide ion ArO-)
smaller Ka/larger pKa
weaker acid