Substitution Reactions!!

...because you can never learn this enough...

A. Nucleophiles

- have unshared pair of electrons or PI bonds, usually a partial NEGATIVE charge, seeking nucleii, "Lewis bases" because are "electron pair donors"

- Nucleophilicity:

1. increases as negative change increases

2. increases going DOWN the period table (I more nuc. than F)

More polarizable, better nucleophile (large radius, can distort the surrounding electrons)

DOWN P.T.

3. increases going left in the period table (NH2- more nuc. than OH-)

Less electronegative, better nucleophile (won't want to pull electrons to itself)

DIAGONAL ACROSS P.T.

- stronger the base, stronger the nuc.

- in protic solvents: larger atoms better nucs

- can shed off solvent molecules better

- in aprotic solvents: more basic atoms better nucs

- naked nucs, more readily donate electrons (F>Cl>Br>I)

B. Electrophiles

- electron deficient, have a full or partial POSITIVE charge, want the nucleophile's electrons

- reaction forms NEW covalent bond

- "Lewis acids" because are "electron pair acceptors"

C. Leaving Groups

- want LG's to be WEAK BASES: (I>Br>Cl>F)

In order for a LG group to leave, it must be able to accept electrons. A strong bases wants to donate electrons; therefore, the LG must be a weak base.

1. more electronegative (accept electrons), better the LG

2. large atomic radius, weaker the base, better the LG

3. Resonance-stabilized structures are WEAK bases, good LGs

D. SN2: simultaneous Nuc addition, LG leaving

- 1 step

- stable in aprotic solvents

ex: DMF/DMSO, hydrogens won't solvate the nucleophile...

- 1°>2°>3°

Reactivity of electrophile (substrate) = less steric hindrance

USUALLY NOT CLUTTERED

- rate = k[Nucleophile][electrophile]

BIMOLECULAR ("2"): a function of 2 variables**

- optically active/inverted product

E. SN1 Reactions: 1) form carbocation, 2) nuc addition

- 2 steps

- stabilized by protic solvents

Hydrogens stabilize the carbocation

- 3°>2°>1°>methyl

Reactivity of electrophile (substrate) = due to stabilization of the carbocation

USUALLY BULKY

- rate = k[electrophile] FIRST ORDER! (b/c of rate-limiting first step)

UNIMOLECULAR ("1"): a function of 1 variable**

- racemic products

- equal amounts of S and R enantiomers of chiral product

- favored with bulky nucleophiles

*solvolysis reaction: solvent behaves as the nucleophile, produces an ALCOHOL product if WATER is used as a solvent.

Use this to identify SN1 rxns quickly!

Fall is in the air. And I'm feeling studious again. And I'm getting over a whole lot that's been long overdue in finding its way out, of my head that is.

DNA Replication

#1. Helicase unwinds helix with help of a topoisomerase forming "negative supercoils" just ahead of it.

#2. SSB's (single-stranded binding proteins) bind to the unwound strands to prevent them from re-annealing.

#3. RNA primase forms RNA primers that bind to the DNA. These are attached by DNA polymerase that zips along in the 5' --> 3' direction (ALWAYS!). Continuous synthesis for the leading strand.

#4. The lagging strand's RNA primers are attached as Okazaki fragments. DNA polymerase zips along in LOOPS (discontinous synthesis) in order to form DNA in the 5' --> 3' direction (opposite that of the leading strand).

- the OVERALL direction of synthesis for the lagging strand, however, is 3' --> 5'

#5. DNA ligase covalently links the fragments.

TAA-DAAAAAAA!!!!!!! DNA HAS DONE IT AGAIN.

Thermodynamics

- Enthalpy (∆H) AND Entropy(∆S) are state functions (only numbers that matter are beginning and end)

- Enthalpy: measure of the TOTAL ENERGY of the system (joules)

Bond Formed = -∆H (energy released)

Bond BROKEN = +∆H (energy absorbed)

- Hess’s Law: summation of enthalpies

- Spontaneity depends on ∆G (∆G = ∆H - T∆S) graphable

∆G will tell you whether a reactions happens or not!

…will tell you NOTHING about rxn rate.

∆G / spontaneity is dependent on temperature, NOT necessarily on ∆H and ∆S.

NEG ∆G = spontaneous

POS ∆G = non spontaneous

ZERO ∆G = equilibrium

∆Gf indicates whether we have a stable or unstable compound relative to its elements.

NEG ∆Gf = STABLE… will spontaneously form compound.

POS ∆Gf = UNSTABLE… will not spon. form compound, need to input energy into the system

POS ∆S = increase in disorder

NEG ∆S = decrease in disorder

Consider why according to eq:

ex: ATP-coupled reactions

ex2: Hydrogen bonding

Apr 09, 2011

Which of the following best describes enthalpy?

(a) Energy that is not available for work in a thermodynamic system (Entropy)
(b) Energy stored in a system derived from its configuration (PE)
(c) The energy of motion in an object (KE)
(d) The total energy of a thermodynamic process (Enthalpy)

uh oh...

So my progress with my MCAT studying this summer is starting to suffer courtesy of road trips and football preoccupations... but all it takes is a few clicks around the internet and a cursory glance at my progress report to briskly scoot my little ass to the kitchen table to forge forward! Viva la estudia!

Rates of Reaction

Learn to always consider reactions in terms of a "decrease in reactants"or an "increase in products" obviously with respect to concentration and the presence of catalysts... helps to think of rate laws and rate-determining steps faster.

5 factors that affect rate: 1) concentration of reactants, 2) temperature, 3) presence of catalyst, 4) rxn medium, 5) surface area of reactants
Observing rate change from physical properties:
(1) COLOR -- measure change in absorption of electromagnetic radiation
(2) Change in PRESSURE w/ gases -- change in number of gaseous molecules

Reaction: aA + bB →(C)→ dD + eE

Rate Law = k [A]^m [B]ⁿ [C]^p

*** DO NOT WRITE THE CO-EFFICIENT for the exponents… they are EXPERIMENTALLY DETERMINED and must be derived from calcuculating the rate order from a table of trials!!!

Caluclating Rate Order:
1. Find trials in which target conc. [A] changes and other conc. variables [B] and [C] stay the same.
2. Note how the rate changes for those 2 trials, and deduce order from rate change:

Rxn order (m):	Rate mult. by:
-1	½
0	1
1	2
2	4

3. Solve for k by plugging in data from ONE trial into the "rate law equation"
Arrhenius Equation: dependence of reaction rate on TEMPERATURE.

k = A e^{-Ea / RT}