Chapter 4: the chair and beyond!
Do substituents prefer axial or equatorial positions on a cyclohexane?
Equatorial
How do you identify a chiral molecule?
Carbon must be attached to four different "things" (this does not just mean atom, it could be attached to four carbons but they are all different like CH3, CH2CH3, COOH, and CH2OH).
*only exception is QUATERNARY ammonium ion
Give an example of an addition reaction.
Double bond + Compound = compound added across double bond.
Give me 5 good leaving groups.
Cl, Br, I, OTs, H2O+
(become Cl-, Br-, I-, -OTs, H2O)
RANK THE STABILITY OF ALKENES! This will probably be an exam question so know all of dem.
The more the merrier. di: GemTransCis
1. Tetra
2. Tri
3. Geminal (di)
4. trans (di)
5. cis (di)
6. mono
Draw the transition state of the cyclohexane ring flip.
Boat.
Consider isobutanol and sec-butanol. Identify whether each one is chiral or achiral.
Isobutanol = achiral
sec-butanol = chiral (has one chiral center)
Is bond breaking exothermic or endothermic?
ENDOTHERMIC you need ENERGY to BREAK the bonds.
What are the main differences between SN1 and SN2 reactions?
SN1: Two steps, weak nucleophile, likes more substituted (tertiary) alkyl halide
SN2: One step, strong nucleophile, likes less substituted (primary) alkyl halide
*secondary can go either way.
What effect does having more substituents on the alpha carbon have on the rate of an E2 reaction?
Increases it. More stable transition state.
What is the difference between configurational and conformational isomers?
Configurational = fixed spatial arrangements; cannot interconvert
Conformational = "conformers", the same compound that interconverts by rotating across a sigma bond
has 2 chiral centers but an internal plane of symmetry makes it an achiral compound overall.
ex) 1,2-dimethylcyclohexane with cis methyls.
Arrange the bonds in order of bond dissociation energy (BDE):
H-CH3
Cl-Cl
H-CH2CH3
H-CH3 > H-CH2CH3 > Cl-Cl (weaksauce)
What indicates a polar protic solvent?
Rank nucleophilicity of the following in a polar protic solvent:
F-
Cl-
Br-
I-
Contain OH or NH and are polar.
ex) H2O, ROH (alcohol), COOH, amides
I- > Br- > Cl- > F-
Normally, strong bases are strong nucleophiles.
Polar protic solvents make strong bases weak nucleophiles (reverse the trend).
Fill in the blanks:
E2 reactions are __________ (endo or exothermic), consuming a ______ base and producing a ______ base.
E2 reactions are EXOTHERMIC, consuming a strong base and producing a weak base.
A cyclohexene underwent an addition reaction to form cyclohexanol. Was the carbon that now has the OH oxidized, reduced, or neither?
(helpful to draw)
Oxidized (-1 to 0)
Consider 1-bromo-2-chlorocyclohexane. Draw a version of it (with wedges and dashes). Then, draw a diastereomer of your molecule.
Only one wedge/dash differs in this case.
A reaction has Δ H° = -42 kJ/mol. What is the Keq for the reaction?
-42 kJ/mol divide by -6 = 7
Therefore, Keq = 10^7
Consider 1-methyl-3-bromocyclohexane, with the groups trans. Draw the reaction with NaCN. Mechanism + products with correct stereochemistry.
SN2 rxn (strong base); 1 step
methyl stereochemistry = same
CN stereochemistry = inverted
Consider 1-iodo-1-methylcyclopentane. Upon reacting with KOH, it makes two products. Draw them, and indicate which is the major product
major = trisubstituted alkene
minor = disubstituted alkene (sticking out from ring)
Draw 1,3-dimethylcyclohexane in the favored chair conformation. Both methyl groups are facing away from you (dashed).
Equatorial favored, both methyls slanting down.
I will draw L-threonine on the board. Star the chiral centers and assign R/S to them.
PLUS
Pure d-threonine has a specific rotation of 24. If a sample of d- and l-threonine was observed to have a specific rotation of 6.0, what is the enantiomeric excess (ee) of the sample?
(drawn)
Star on C attached to OH and C attached to NH.
R under OH, S on NH.
6/24 * 100= 0.25 *100 = 25% ee
Consider: CH4 + Cl2 --> CH3Cl + HCl. Is the reaction exothermic or endothermic? Draw the rxn energy diagram!
BDEs:
H-CH3 = 435 kJ/mol
Cl-CH3 = 351 kJ/mol
Cl-Cl = 242 kJ/mol
H-Cl = 431 kJ/mol
Bonds broken (+): total = +677
H-CH3 = 435 kJ/mol
Cl-Cl = 242 kJ/mol
Bonds formed (-): total = -782
Cl-CH3 = 351 kJ/mol
H-Cl = 431 kJ/mol
Put them together: +677 + (-782) = -105 kJ/mol
EXOTHERMIC. Energy of products lower than reactants.
Draw the reaction mechanism between 2-chlorobutane and H2O.
What does doubling the concentration of H2O do to the rate of reaction?
SN1 (weak base); two step
first: Cl leaves. carbocation forms
second: H2O attacks
cleanup: Br- takes away extra H.
Draw the reaction mechanism of 3-bromo-2,3-dimethylpentane with LiCN. Give only the major product.
the tetrasubstituted alkene. 2,3-methylpent-2-ene