L.6 Carbohydrates & Lipids Flashcards
Lewis Acid
- Electron Pair Acceptors
- Electrophiles
- Vacant orbitals or
- positively polarized atoms +
Lewis Bases
- Electron Pair Donors
- Nucleophile
- lone pair of electrons can be donated
- Anions carrying - charge
Amphoteric molecules?
Four molecules?
Can act as either acids or bases, depending on reaction condition.
Act as an acid in basic solution
Act as a base in acidic solution
H2O
Al(OH3)
HCO3-
HSO4-
What is the acid dissociation constant
Ka = is a measure of acidity
It is the equilibrium constant corresponding to the dissociation of an acid HA into a proton H+ and its conjugate base A-
pKa
pKa is the negative logarithm of Ka.
pKa smaller = More Acidic
- 2 & below = strong acid
- 2 to -20 Weak
pKa decreases down the periodic table and increases with electronegativity
FOUR Functional Groups that act as Acids
- Alcohols
- Aldehydes
- ketones
- carboxylic Acids
and derivatives;
anhydrides
esters
Two Functional Groups that act a bases
- Amines
- Amides
Nucleophiles
Nucleus Loving contain lone pair of electrons or pi bonds
increase in electron density and often carry a negative charge.
- Nucleophilicity = Basicity; however nucleophilicity is a kinetic property, while basicity is thermodynamic.
- Charge, electronegativity, steric hindrance, and the solvent can all nucleophilicity
- amino groups are common organic nucleophiles.
Electrophiles
Are electron loving and contain positive change or are positively polarized
more positive compounds are more electrophilic
alcohols, aldehydes, ketones, carboxylic acids and derivatives.
What are Leaving Groups?
- Are molecular fragments that retain electrons after heterolysis
- Opposite of coordinate covalent
- Best leaving groups can stabilize additional charges through resonance or induction
- Weak bases (the conjugate base of strong acids) make good leaving groups. (I- Br- Cl-)
- Alkanes and Hydrogen ions are almost never leaving groups because they form reactive anions
SN1 Unimolecular nucleophilic substitution reaction
Name the 4 Steps &
On what Carbons do they react.
2 steps (1st step rate limiting)
- 1st step = leaving group leaves
- Carbocation forms
- Nucleophile attacks (attacking planar molecule from either side)
- Racemic mixture
- Prefer more substituted carbons because the alkyl groups can donate density and stabilize the positive charge of the carbocation.
- Rate-dependent only on the concentration of the substrate.
CARBONS;
1 & methyl = always
2 = sometimes
3 never
SN2 Bimolecular nucleophilic substitution reaction
One step concerted reaction
- Nucleophile attacks at the same time the leaving group leaves
- must be a backside attack which leads to an inversion of stereochemistry
- the absolute configuration is changed R to S and vice versa- if the incoming nucleophile and the leaving group have the same priority in their respective molecules.
- rate dependent on substrate and nucleophile
- RATE = k [Nu] [R-L]
CARBONS
1 and methyl = always
2 sometimes
3 never (steric hindrance)
Good Nucleophiles
(BEST)
HO- RO- CN- N3-
(GOOD)
NH3 RCO2-
(OK)
H2O ROH RCOOH
Amines (functional group)
Protic Solvent VS Aprotic solvent
PROTIC; Can hydrogen Bond, ammonia, amines, water, alcohol, carboxylic acids.
APROTIC; cannot hydrogen bond, DMF, DMSO, Acetone
Polar solvents can dissolved nucleophiles
Order of Functional groups by average pKa
Alkane 50
Alkene 43
Hydrogen 42
Amine 35
alkyne 25
Ester 25
Keytone/aldehyde 25
Alcohol 17
Water 16
Carboxylic acid 4
Hydronium Ion -1.7
Oxidation State
The Oxidation state of an atom is the charge that it would have if all its bonds were completely ionic.
CH4 C = - 4 Most Reduced
CO2 C = + 4 Most Oxidized
Carboxylic acids and its derivatives are the most oxidized followed by aldehydes, ketones, and imines, followed by alcohols, alkyl halides, and amines.
Oxidizing Agents
3 & 4
ACCEPT ELECTRONS
O2 O3 Cl2
(Have a high affinity for electrons)
Mn7+ MnO4- Cr6+ CrO42-
(Unusually high oxidation states)
1* Primary Alcohols become oxidized to aldehydes by PCC and to Carboxylic Acids by CrO3 Na2Cr2O7K2Cr2O7
2* Secondary Alcohols become oxidized to ketone.
Reducing Agents
4 & 4
DONATE ELECTRONS
Na. Mg. Al. Zn
(Low electronegativities, low ionization energies, left of the periodic table)
NaH. CaH2. LiAlH4. NaBH4.
(Metal hydrides, H= Ion which helps reduce)
- Aldehydes, Keytones & Carboxylic Acids can be reduced to alcohols by LiAlH4
- Amides can be reduced to amines by LiAlH4
- Esters can be reduced to a pair of alcohols by LiAlH4
Amides VS Amines
What is chemoselectivity?
- Both nucleophile-electrophile and oxidation-reduction reactions tend to act on the HIGHEST PRIORITY (MOST OXIDIZED) FUNCTIONAL GROUP
- Once can use STERIC HINDRANCE properties to selectively target functional groups that might otherwise not react, or to protect functional groups and prevent them from reacting.
- Diols* protecting groups for aldehydes or ketone carbonyls.
- Alcohols* protecting by conversion to tert-butyl ethers.
6 Steps for Problem Solving
- Know Your nomenclature
-
Identify the functional groups
- Act as acid or base
- How oxidized is the carbon
- Good Nu or Elec or L.G.?
-
Identify other reagents
- Acidic or Basic?
- Suggestive of a particular rxn?
- Good Nu or Specific solvent?
- Goos oxi or red agents?
-
Identify the Most reactive functional groups
- Also, notice protecting groups
-
Identify the first step of the reaction
- Acid/base… then Protonation/deprotonation
- Nu…..then Nu attack forming bond
- ox/re agent… then most oxidized f.g. will get ox/re
-
Consider Stereoselectivity
- Stereospecificity SN2
- Stereoselectivity “Stability of product… Conjugation or torsional strain.
