L.6 Carbohydrates & Lipids Flashcards Preview

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Flashcards in L.6 Carbohydrates & Lipids Deck (21)
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1

Lewis Acid

  • Electron Pair Acceptors
  • Electrophiles
  • Vacant orbitals or  
  • positively polarized atoms +

2

Lewis Bases

  • Electron Pair Donors 
  • Nucleophile
  • lone pair of electrons can be donated
  • Anions carrying - charge

3

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-

4

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-

 

 

 

5

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

6

FOUR Functional Groups that act as Acids

1. Alcohols 

2. Aldehydes

3. ketones

4. carboxylic Acids

 

and derivatives; 

anhydrides 

esters 

 

 

7

Two Functional Groups that act a bases

1. Amines

2. Amides

8

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.

9

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.

10

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

11

SN1 Unimolecular nucleophilic substitution reaction

Name the 4 Steps &

On what Carbons do they react.

2 steps (1st step rate limiting)

  1.  1st step = leaving group leaves
  2. Carbocation forms
  3. Nucleophile attacks (attacking planar molecule from either side)
  4. 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 

 

12

SN2 Bimolecular nucleophilic substitution reaction

 

One step concerted reaction 

  1. Nucleophile attacks at the same time the leaving group leaves
  2. 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)

13

Good Nucleophiles

(BEST)

HO-  RO-  CN-  N3-   

 

(GOOD)

NH3  RCO2-  

 

(OK)

H2O ROH RCOOH 

 

Amines (functional group)

14

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

15

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

 

 

16

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.

17

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    Na2Cr2O7    K2Cr2O7

2* Secondary Alcohols become oxidized to ketone.

 

 

18

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

19

Amides VS Amines

20

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.

21

 6 Steps for Problem Solving

  1. Know Your nomenclature
  2. Identify the functional groups
    • Act as acid or base
    • How oxidized is the carbon
    • Good Nu or Elec or L.G.?
  3. Identify other reagents
    • Acidic or Basic?
    • Suggestive of a particular rxn?
    • Good Nu or Specific solvent?
    • Goos oxi or red agents?
  4. Identify the Most reactive functional groups
    • Also, notice protecting groups
  5. 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
  6. Consider Stereoselectivity
    • Stereospecificity SN2
    • Stereoselectivity "Stability of product... Conjugation or torsional strain.