CHEM 0310 Lecture Notes - Lecture 2: Cyclopentane, Water Chlorination, Bromine

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O Chem Unit 2:
Other Radical Halogenations: look more closely at the 2 propagation steps
P1 Xdot +CH4 → CH3dot +HX
P2: CH3dot + X2 → CH3X + X
If X were, F, Cl, Br, or I step 1 would be -31 (exothermic), +2, +18, +34 endo respectively.
Generally: Bonds broken- bonds made→ enthalpy of rxn.
In P1: breaking C-H bond, making H-X bond.
(C-H)=105 --- 136 (H-X) =-31 kCal/mol use data in book to justify.
Step 2: -103, -25, -6, +13 respectively.
F: -103 is so exothermic you can’t control it so not useful.
I : +13 is way to slow, also not useful.
So Cl and Br are the only useful ones for a reaction like this that we actually want to control.
Reaction diagram:
Progression of reaction for P1
Progression of reaction for P2
-------------------
Hammond postulate: early transition state resembles starting materials, late transition state
resembles products.
Late transition state means the rxn forms the radical quite a bit which means its more stable.
Bromine is selective for secondary carbons.
If have substrate with only one type of C on it (cyclopentane for ex) then you don’t have to
worry about selectivity b/c all C are the same (do have to control for over-chlorination)
Sec 3-7 Chlorination of Higher Alkanes
How many H are there to extract (statistical analysis)?
Which radical is more stable?
Chlorinating Propane:
CH3CH2CH3 +Cl2→ (hv) → CH3CH2CH2Cl (primary) + CH3CHClCH3 (secondary)
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6 primary Hs and 2 secondary Hs so primary favored 3:1
At 25 C
So STATISTICALLY expect 75% products to be primary, 25% secondary
But STABILITY expect less primary and more secondary
EXPERIMENTALLY: 43% primary, 57% secondary
Tertiary C:
Cl2 + HC(CH3)3 → (hv) → Cl CH2-C(CH3)2H + ClC(CH3)3
STATISTICALLY: 9:1 in favor of primary 90% primary 10% tertiary
STABILITY: less primary more tertiary
EXPERIMENTALLY: 64% primary 36% tertiary
LESS REACTIVE primary<secondary<tertiary MORE REACTIVE
1: 4: 5 odds
Fluorine:
F2 +(CH3)3CH → (hv) → FCH2(CH3)3CH + (CH3)3CF
STAT: 9:1
STAB: less primary more tertiary
EXP: 86% primary 14% tertiary
B/c transition state of fluorine forms very close to the reactants so the radical doesn’t form to a
great extent to radical stability doesn’t have as much effect so we see numbers closer to
statistical prediction.
(MAKE A LIST DEVOTED JUST TO THE REACTIONS WE’VE LEARNED AND JUST TO
THE MECHANISMS)
Ch4: Cycloalkanes: Formula: C2H2n
Any ring has IHD of 1
If has ring AND double bond then 2
Cyclopropane (triangle)
Cyclobutane(square)
Cyclopentane (pentagon)
Cyclohexane (hexagon)
REMEMBER these are all sp3 hybridized so NOT actually flat, remember what they would
really look like in space.
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Rules for naming: monosubstituted substrate (1 substituent on it), then C with substituent is C
one (C1).
If 2+ substituent: give lowest numbering possible. Ex: 1-chloro-2 methyl-4-propyl cyclopentane.
Isomers: can get cis/trans isomers (doesn’t have to be on adjacent C)
Remember 3D view of it, if both substituents up on adjacent carbons, then cis, if one up one
down then trans.
INCLUDE IN NAME
Ex: cis 1-bromo-2-chloro cyclobutane vs trans-1-chloro-2-bromo cyclobutane
Melting points, Boiling points, Density all greater than those of the straight chain alkanes
Cyclic compounds can pack closer together
Ring strain:
The more ring strain/angle strain there is the less stable than cyclohexane.
The further away from 109.5 degree bond angles the less stable.
So cyclopropane has very sharp angles, more strain than butane.
Measure strain using heats of combustion:
But can’t directly compare propane (C3H8) and cyclopropane (C3H6) b/c an extra CH2 group
SO get multiple known heats of combustion for straight chain alkanes in Kcal/mol.
Propane: -530.6
Butane: -687.4
Pentane: -845.2
Hexane: -1002.3
The difference between propane and butane is ALSO a CH2 group, so see how much the values
differ and that's the value a CH2 group makes.
So -687.4- (-530.6) =-156.8 and can do calcs for others as well.
~-157.4 kcal/mol per CH2 group
So cyclopropane (C3H6) is just a chain of 3 CH2 groups connected at the ends so 3(157.4)= we
EXPECT -472.2 Kcal/mol
BUT it's actually 499.8 Kcal/mol BECAUSE OF RING STRAIN and the difference between
actual and expected is the exact value of RING STRAIN.
That difference should decrease as you go up to butane, pentane etc b/c the angels get closer and
closer to 109.5. Cyclohexane expected=-944.4 and actual is -944.5 BASICALLY THE SAME.
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