CHEM 201 Exam 2

Newman Projections

front and back carbons

rotate the back carbon when changing by 60°, go up to 360°

with single bonds there’s free rotation

with double bonds there’s no rotation

helps to draw the line angle formula first: there are 2 base drawings where the in plane atoms slightly change

changing the angle = dihedral angle

when the atoms are aligned draw a short line next to the front carbon atoms

staggered conformation (more stable): atoms/lines are not aligned, lower energy

• gauche where the big groups are only 60° apart, less stable

• anti where the big groups are farthest away (180°), most stable so lowest energy

eclipsed conformation: atoms/lines are aligned so less stable and higher energy

• totally eclipsed (the biggest group is in front of the other big group): very unstable so highest energy

Cyclohexane conformations

Chair: most stable, 4 carbons in plane, parallel lines aren’t perfectly aligned, axial goes up and down, equatorial goes away and points away from ring

Boat: has flagpole hydrogens (2 H’s at the top) results in electron repulsion (unfavorable), less favorable conformation

Twist boat: more stable than boat, like a square figure 8

Half chair: the least stable, 5 H’s are in plane and 1 H is out of plane so there’s more torsional strain and is more eclipsed

Order of conformations in increasing order/decreasing stability: Chair << twist boat < boat << half-chair

Ring interconversion: axial → equatorial, equatorial → axial (only thing that stays up is the direction so up stays up and down stays down), equatorial positions are more stable

• 1,3-diaxial interactions is when a substituent is axial up and has interactions with other hydrogens that are axial as well

The bulkier/larger the substituent the more stable the equatorial conformation will be

Drawing cis and trans in chair conformation: if it’s towards you aka a solid wedge then it’s upward/above the plane, if it’s away from you aka a dashed wedge then it’s downward/below the plane

• when after rotating both conformations have 2 methyl groups with one axial and the other equatorial then it’s the same energy

Cycloalkanes

shapes it can be: triangle, square, pentagon, hexagon

C_nH_2n: when you form a ring, you lose 2 H’s

Nomenclature: identify cycloalkane then its substituents, the carbon with the substituent = C #1 (if just 1 substituent then don’t have to put number bc it’s implied it’s on the first), remember to write in alphabetical order (for alphabetical the prefix doesn’t have an impact), when there’s more substituents on a carbon then it has the higher priority for numbering the molecule, 2 substituent neighbors will be C #1 and #2

Cis-trans isomers: disubstituted cycloalkanes, if both substituents point the same way either towards or away then it’s cis, if one is towards you and the other is away then it’s trans

Ring Strain: measures stability made up of angle train and torsional strain

• cyclopropane (triangle) = least stable, angles are 60° but ideal is 109.5° so angle strain causes poor orbital overlap, torsional strain all hydrogens are eclipsed

• cyclobutane (square) = 88° angle (ideal = 109.5°) lowers stability but also reduced torsional strain so stability actually increases and has a stronger impact than angle strain

• cyclopentane (envelope shape): bond angles is about 108° (ideal is 109.5°), torsional strain is not planar so it reduces torsional strain bc envelope makes it so 1 atom always out of plane

• cyclohexane: most stable, no ring strain bc bond angles are 109.5° and there’s no torsional strain bc no eclipsing interactions

Isomers

compounds with the same condensed formula

Constitutional/structural: different connectivity

Stereoisomers:

• Enantiomers (chirality)

• Diasteromers

  • geometric (cis-trans)

  • 2 or more chiral centers

*for line angle ignore perspective, don’t put wedged and dashed, put wedged and dashed for perspective

Chirality

chiral objects have a mirror image but it’s different from the original (non-superimposable) so they’re a pair of enantiomers

• enantiomers: when there’s no plane of symmetry it’s likely to be chiral

achiral: the original is = to the mirror image, they’re the same (no enantiomers)

Chiral Centers

sp³ atoms with 4 different electron domains

a molecule with 1 chiral center is chiral

a molecule with 2 or more chiral centers then:

• some are achiral

• some are chiral

when there is a plant of symmetry then the molecule is achiral

N and P can also be chiral centers (enantiomers can be achiral when they are 1:1 and the reaction happens fast)

Optical activity

polarizing light → looking only at light with a specific angle

light goes in all directions but if it travels through polarizing filter that polarizes the light and then that passes through a chiral compound then you will have a certain angle depending on the chiral compound

• angle = +, clockwise, then labeled as d or + (d = dextrorotatory)

• angle = -, counter-clockwise, then labeled as l or -, (l = levorotatory)

it’s used bc chiral compounds are identical except for their optical rotation: bp, mp, density, and refractive index are all the same and the optical rotation is the only different thing

chiral molecules rotate the plane of polarized light = optically active

achiral molecules don’t rotate the plane of polarized light = optically inactive

[α]²⁵_D = specific roatation: 25 = temp, D = light source/D line of sodium spectrum (assume these are the conditions if it doesn’t say anything else)

each chiral compound has its own specific rotation

[α]²⁵_{D =} optical rotation (α) divided by (c x l)

• α = experimental rotation, c = concentration g/mL, l = length of the cell in dm (1 m = 10 dm, 1 dm = 10 cm, 1 dm = 100 mm)

• when calculating this all the units cancel out except for degrees °

Enantiomeric excess (e.e.)

e.e. = optical purity x 100

α/[α]²⁵_D = optical purity

(α/[α]²⁵_D) x 100

other way to get e.e.: (|[d]-[l]|)/([d]+[l]) all multiplied by 100

e.e. is the molar ratio of enantiomers

R and S configuration

1) assign 1-4 priorities

• 1 = highest priority aka highest atomic #, 4 = lowest priority

• do tie breakers if needed

• treat double and triple bonds as if each were a bond to a separate atom

2) Draw a curved arrow to connect groups 1-3

3) If group 4 is pointed away then clockwise = R and counterclockwise = S, but if group 4 is not away then you must swap 2 pairs of groups so that H is away from the viewer and then identify as R or S

in molecules that have a sigma plane of symmetry it’s the opposite so if a chiral center on one side is the R configuration then the other is the S configuration

when there’s a double bond and you’re doing tie breakers, write it twice as though the molecule is bonded twice so then you can get priority numbers

Meso compounds

when there are 2 or more chiral centers with a sigma plane of symmetry (they are also achiral)

Diastereomers

stereoisomers that are not mirror images

• have 2 or more chiral centers

stereoisomers = diastereomers + enantiomers

switch all chiral centers to get enantiomers

don’t switch all chiral centers to get diasteromers

# of stereoisomers that a molecule has = 2ⁿ_, n = # of chiral centers

Fischer projections

doesn’t show stereochemistry

vertical line and horizontal line making a cross

vertical = away always, horizontal = towards you always

90° rotation = enantiomers

180° rotation = same

types of organic reactions

addition (only add atoms, not remove)

elimination (getting rid of atoms for a pi bond)

substitution (Nu + R—X → R—Nu + X)

rearrangement (migrating groups from 1 to another)

Polar mechanisms (nucleophile + electrophile)

negative attacks the positive (always more negative to more positive)

• if both are neutral then the one with lone pairs will attack

Nu: → E⁺ = unsymmetrical bond forming

E—X → X: + E⁺ = unsymmetrical bond breaking

unsymmetrical because 1 atom has the electrons and the others don’t

Nu: = neutral or negative (not positive)

• pi bonds can also act as nucleophiles, O, N, C^-

Electrophiles:

• R—X (halogen), OH group, carbonyl

Radical mechanisms

symmetrical bond breaking: C—H → C. + .H

symmetrical bond making: C. .H → C—H

Bond dissociation energies

the energy needed to break chemical bonds, breaking chemical bonds is endothermic

∆H = (-) exothermic ∆H = (+) endothermic

when products are lower in energy than reactants, it’s favorable because it’s more stable

∆H_rxn = BDE_broken - BDE_formed

stronger bonds = lower energy

when bonds formed are lower in energy then they are stronger than bonds broken

thermodynamics talk about stability

Kinetics (speed)

Rxn rate = k x [R]ⁿ

k = rate constant

k_r = Ae^-E_a/RT

A = constant, frequency factor, where molecules have the right orientation and energy

E_a = activation energy, the higher it is the slower the rxn, not changed by temperature, increasing temperature helps in preparing more molecules

transition state: shows how chemicals are being broken and formed (each step/E_a has a transition state)

rate determining step:slowest step, has the highest E_a

Degrees of unsaturation (D.U.)

*pi bonds are faster to react than sigma bonds since sigma bonds are stronger and more stable

D.U. = the amount of times 2 H’s can be added (pi bonds can react to then add H’s)

• each degree corresponds to 2 fewer hydrogen atoms than the saturated form has

D.U. = # C’s - ((H’s-N’s+X’s)/2) + 1

Alkene Nomenclature

ending = -ene

1) find the longest chain or largest ring with the largest number of double bonds

2) number the chain from the end that’s closes to the double bond

3) name substituents

4) include where the double bond starts (ex: hept-2-ene)

5) add cis-, trans-, E-, or Z- where appropriate

for rings: if there’s only 1 double bond and 0 or 1 substituent than you don’t have to specify where the double bond is because it always is on carbon 1 and 2

for rings: when there are 2 or more substituents, go in the direction towards the side with more substituents

when there are 2 or more double bonds you need to act the -a to the first part of the name (ex: hexa-1,3-diene)

Cis-trans and E/Z alkenes

cis: when the higher priority groups are on the same side of the imaginary axis (line parallel to double bond)

trans: when the higher priority groups are on different sides of the imaginary axis

E/Z: is used when there are 4 different substituents (look at higher priority groups, E = trans, Z = cis)

• if a compound has 1 alkene with 4 different substituents but another alkene doesn’t have 4 different substituents still treat it as if they do and name both using (E/Z)

Stability of alkenes

cis vs. trans: trans is more stables than cis because its ∆H_rxn is < the cis ∆H_rxn because with cis more energy is released and the start material is less stable

hyperconjugation: look at notes for the trend from most stable to least stable, when p orbitals are empty then electron density can be donated which increases the stability and hyper conjugation stabilizes the carbocation

• 3° carbocation is most stable (has 3 carbons attached)

• resonance also give extra stability because it’s better for more atoms to carry the charge

Electrophilic addition reactions of alkenes

1. attack of the pi bond on the electrophile (formation of carbocation)

2. nucleophilic attack

the electrophile accepts electrons and the anion attacks the carbocation

Addition of HX

pi bond will attack H, the bond between HX will go to X and then X^- will attack the carbocation

markovnikov’s rule: the addition of a proton to the double bond of an alkene results in a product with the acid proton bonded to the carbon atom that already holds the greater number of H atoms (basically H goes to the side that has more Hs)

Hydration of alkenes (addition of water)

opposite of dehydration

reaction mechanism:

1. protonation of the acid (this is reversible) so what happens is the pi bond attacks the acid and forms a carbocation

2. then O in water attacks the carbocation but this then makes the O positive

3. then there is a deprotonation where another water molecule will attack one of the hydrogens attached to O⁺ and then that bond will go to O and O will now be neutral

*leave rings alone, chemistry happens on isolated alkene

The hammond postulate

we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species

endothermic reactions: have product-like transition states, transition state is closer to the product so their energy difference is very similar, it has a larger difference between the E_a of both products and a large difference in ∆H, a big difference in E_a is good because the reaction is selection (you can make more of the stable product because it forms faster and because of that big difference), when it’s a small difference there isn’t selectivity

exothermic reactions: have reactant-like transition states, no difference in reactants so there’s a small difference in E_a because the TS is closer to the reactants, large difference in ∆H

the more stable the carbocation intermediate the faster the carbocation formation

when drawing out the bonds being broken and the bonds being formed, when the TS is product-like then there will be a shorter distance between the product and a longer bond between the reactant

Synthesis of Alkenes

1. dehydrohalogenation (H—X removal that forms an alkene)

• alkyl halide —base+heat→ alkene

• the base removes the H that’s on the neighboring C of the X and then that bond creates the double bond (it doesn’t happen on the same C as the X)

2. dehydration of alcohols

• alcohol —H⁺(acid)+H₂O+heat→ alkene + H₂O

• rxn mechanism: the OH group electron attacks H⁺ which is reversible also turns O positive and then the base aka H₂O attacks the neighboring H which forms the double bond and then then old OH bond is broken and O goes from positive to neutral

Rxns of alkenes

1. addition of HX (markovnikov product)

2. hydration of alkenes (markovnikov alcohol)

3. addition of halogens to alkenes

• when adding X₂ the Xs are added so that one is up and one is down and it’s called the anti addition