Home > Chemistry, Organic chemistry > Chemistry 51A Lecture 19 Notes 11/20/09

Chemistry 51A Lecture 19 Notes 11/20/09

Overview

Today is a 51 minute lecture on stereochemistry and mechanisms of SN2 reactions.

Details

Inversion of stereochemistry at the sp3 center.  An alkyl halide is a nucleophile.  Halogens are more electronegative than carbons.

The nucleophile comes in 180 degrees behind, inverting the stereochemistry at the center. It is important to be able to visualize what is going on.

N3- is azide.  The negative charge is very spread out and delocalized.

The transition state is trigonal bipyramidal.  We go from cis to trans.

Electrophiles are RX in SN2 displacement reactions.  The general idea is that good electrophiles are not sterically hindered.  This is why primary alkyl halide reactions undergo SN2 reactions.

CH3X is a great electrophile.  Same for allyllic halides like CH2CHCH2X.  Benzylic halides are good – C6H5CH2X.

I>Br>Cl in term so leaving group ability.

RCH2X>R2CHx>R3CX for SN2 reactions.  Steric hindrance prevents backside attack.  Branching also slows the SN2 reaction way down.

CH3O-Na+ (sodium methoxide) + C6H5CH2Cl (benzyllic halide) = C6H5CH3OCH3 + NaCl

CH3O-Na+ (sodium methoxide) + C6H5Cl =  No reaction.  sn2 center.  Chlorobenzene is not an electrophile.  No SN2 displacement as the bonds are too strong.

CH3O-Na+ (sodium methoxide) + Tertiary AH =  No reaction.  Steric hinderance.

Time for secondary alkyl halides.  With good nucleophiles, mostly SN2 displacement reactions occur.  Strong bases yield mostly elimination reactions.

CH3CHBrCH2CH3 + CH3CH2S-Na+ = CH3CH(CH3CH2S)CH2CH3

CH3CHBrCH2CH3 + CH3CH2O-Na+ = mostly elimination

Common nucleophiles have electrons and a willingness to share them.  A lone pair helps.  RO-, RS-, X-, CN-, RC-=C-, H2O, R2S

Sulfur has a large polarizable cloud.  More polarizable is better, more reactive, less likely to participate in elimination reactions.

RS- > RO-

R2S, RSH, H2S > R2O, ROH, H2O

For One Atom Type:

More basic, better nucleophile.

RO- > RCOO- > ROH

Less steric hinderance, better nucleophile.

CH3O- > tert buyt oxide

Review

X = fluorine, chlorine, bromine, iodine, astatine (radioactive, no stable isotopes)

Ambush – deploy, suppress, report

n1sin01 = n2sin02

SN2 = inversion of stereochemistry

Nice-Manipulative-Assertive-Aggressive-Assaultive-Murderous

L Chatlier – add reactants/products, volume, pressure, temperature

Successful Ambush: Surprise , Speed, Violence of Action

Aging: oxygen, UV light, water, heat

A = a l c

Electronegativity: NOF Cl Kr

Alkane CnH2n+2

Alkene CnH2n

Alkyne CnH2n-2

Most important cyclic molecules: Benzene C6H6/Naphthalene C10H8

Moving formations: column line wedge vee eschelon

When not moving: herring bone and coil

Transversing Terrain: traveling, traveling overwatch, bounding overwatch

breaking bonds takes energy, making bonds releases energy

Loads on average were: – fighting load: 63 lbs – approach march load: 101 lbs – emergency approach march load: 132 lbs

One well-established norm is that a soldier cannot sustain a load greater than one-third of his body weight over time.

Noble gases: He, Ne, Ar, Kr, Xe, Rd

Types of bonding: ionic >1.7, polar covalent between, non polar <0.5

PV = nRT

METT TC mission enemy troops terrain time civilians

SALUTE size activity location unit time equipment

2s 6p 10d 14f

KE electron = E photon – Ionization Energy

Tissue: epithelial, connective, nervous, muscular

Epithelial: simple/stratified/pseudostratified cuboidal/columnar/squamous

  • single covalent bond – 1 shared pair
  • double covalent bond – 2 shared pairs
  • triple covalent bond – 3 shared pairs

Bond energy: single < double < triple

Bond length: single < double < triple

pKa’s:

Alkanes (50)

CH4/CH3-

Alkene (44)

Alkyne (25

HF +3.2

HCl -7

HBr -9

HI -10 NH3 38 L

RCOOH (4-5)

HF (3.2)

NH4+ (9.26)

H2O (15.7)

ROH (17)

RSH (11)

H30+ (-1.7)

pKa = – log Ka

Spectroscopy: emission/absorption

A = e l c

IGL: point source, random motion, elastic collisions, high temp, low pressure

6 strong acids:

  • HCl
  • H2SO4
  • HNO3
  • HClO4
  • HBr
  • HI

The stronger the acid, the higher the Ka, the weaker the conjugate base.

hydrogen helium

lithium beryllium boron carbon nitrogen oxygen fluorine neon

sodium magnesium aluminum silicon phosphorus sulfur chlorine argon

potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton

rubidium strontium yttrium zirconium niobium molybdenum technicium ruthenium rhodium palladium silver cadmium indium antimony tin tellurium iodine xenon

cesium barium lutium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon

SWAT – snipers, entry team, inner perimeter/gas people

AOR = area of responsibility

Response to ambush: Lay down a base of fire, maneuver, indirect-fire support, and close-air support.

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