Module 8 The Chemistry of Alcohols Flashcards
Alcohols posses a ______ ______.
Hydroxyl Group (-OH)
Like Halides Alcohols are classified by the type of _____ they are attached to.
Carbon.
Primary, Secondary , Tertiary.
Alcohol attached to a benzene ring.
Phenol.
Physical Properties of Alcohols.
Higher Boiling Point due to to H-Bonding.
Acidity of Alcohols and Phenols.
A strong base is necessary to deprotenate an alcohol.
NaH is often used to generate the corresponding alkoxide.
Alkoxide.
Conjugate Base of an Alcohol.
Which is more acidic Phenol or Cyclohexanol?
Phenol because its conjugate base is stabilized by resonance.
Induction and acidity.
Presence of electron withdrawing groups increases the acidity of an alcohol or phenol.
Solvation and Acidity.
The more poorly solvated a conjugate base, the less stable it is and the less acidic it’s conjugate acid.
more sterically hindered the molecule the less able the solvent is to stabilize th charge.
Alcohols can be synthesized from alkyl halides via ______ and alkenes via ______, using what reagents?
Substitution:
SN1: H2O (secondary;tertiary)
SN2: NaOH (primary)
Addition:
H2SO4 (dilute/ with water)
BH3
A third method for generating alcohols.
The reduction of Carbonyl compounds (C=O)
Converting a carbonyl to an alcohol requires a ________ agent.
Reducing Agent.
It is oxidized in the process while the carbonyl is reduced.
The reduction of the carbonyl group, overall, results in the addition of H-H across the pi bond.
Carbonyl Properties.
Has Resonance
The carbonyl carbon is very electrophilic, it doesn’t have electron density.
Quick Note on determining Oxidation States.
To determine the oxidation state of an atom, imagine the electrons in a bond as a lone pair on the more electronegative atom
Oxidation RXN vs Reduction RXN.
Ox rnx: Carbon’s oxidation state is increased (0 to +2). Loss of electrons.
Red rxn: Carbon’s oxidation state decreases (+2 to -1) Gaining electrons.
Catalytic Hydrogenation.
H2, Pt, Pd or Ni.
Adding Hydrogen across the double bond.
This method is rarely used; high temperature and pressure is required.
Also not very selective hydrogen will add across any double bond present.
Sodium Borohydride (NaBH4)
Common Reducing Agent for aldehydes and ketones.
Hydrogen in borohydride reacts as if it is an H-.
The H- is a good nucleophile that can attack the carbon, leaving oxygen with a formal negative charge.
This oxygen is then protenated giving the final product.
Lithium Aluminum Hydride.
LAH
Hydrogen behaves like H- and attacks the carbonyl carbon.
Electrons will go to the oxygen, then a proton donor H2O is added in the second step to reduce the negatively charged oxygen.
The Al-H bond is a much stronger H- donor because of that we can’t have any acidic hydrogen in the reaction when we add the aluminium hydride, it will react as an acid base reaction.
Which is more reactive LAH or NaBH4
Aluminium is much less electronegative than boron, the bond between Al-H is much more polarized. LAH is much stronger H- donor than BH4.
BH4 is a much weaker H- donor so we can use H2o or alcohol as a solvent.
LAH can’t have H2O or OH as a solvent.
When ______ ketone is reduced a new _____ ____ is created and a pair of ______ is obtained.
Asymetrical.
Chiral Center
Stereoisomers.
SP2 hybridized flat carbonyl compound, H- nucleophile can attack from either face of the carbonyl group.
Key difference between Hydride delivery (LAH;NaBH4) and hydrogenation.
Hydride delivery agents are selective for the carbonyl group whereas hydrogenation is not.
Catalytic hydrogenation reduces the carbonyl and the alkene.
Note about hydride delivery reactivity.
Can be fine tuned by using derivatives with varying R-groups.
changing the R group makes the hydride either more electron withdrawing or donating, more or less reactive.
_____ is strong enough to reduce ester and carboxylic acids whereas ____ is generally not.
LAH.
NaBH4.
Reduction of carboxylic acids and esters yeild primary alcohols.
Reduction of Esters.
- Requires 2 equivalents of hydride.
- the first two steps result in the formation of an aldehyde, which is then reduced to an alcohol.
- Methoxide is nor a good leaving group, but acts like one here because the high energy intermediate becomes a stable aldehyde.
How are Grignard Reagents formed?
- To form a Grignard Reagent an alkyl halide is treated with Mg.
- Mg inserts itself between the halide and the carbon.
- Carbon now behaves like C- because carbon is more electronegative than magnesium.
- A carbon with a formal negative charge is a good base and nucleophile.
- Reactions are performed in ether because we can’t use C- in the presence of a proton source,