CH 220C BROMINATION QUIZ

what is an arene?

an aromatic hydrocarbon

what reaction type are we studying during the bromination of arenes lab?

radical bromination through a free-radical substitution

what are we investigating in the arenes lab?

the rate at which C-H bonds in different molecular environments undergo radical bromination

though CH bonds are typically unreactive, they act as _________ ________ in reactions that employ radical halides

functional groups

what are the reagents of the arenes experiments?

compound + Br2 + light

what are the 4 compounds we are testing? **draw these on paper & circle the C-H bonds that will be broken and reformed with Br**

toulene, ethyl benzene, isopropyl benzene, terbutyl benzene

what is the difference between primary, secondary, and tertiary carbons?

primary (RCH3), secondary (R2CH2), tertiary (R3,CH)

how do you know which C-H bond the Br radical will attack?

it will attack the one that gives it the most radical stability due to hyperconjugation

are primary, secondary, or tertiary carbons more reactive with/ preferential to bromine radicals and why?

3 > 2 > 1* because Bromine prefers to attack C-H bonds that will give the radical more stability (similar to carbocation stability) due to hyperconjugation

what is hyperconjugation?

spreading out charge by the overlap of an empty p orbital with an adjacent sigma bond

CH3 groups donate e- density to stabilize the missing e- density of the radical

what are benzylic, aromatic, and aliphatic C-H bonds which are most reactive?

benzylic: H bonded to sp3 C bonded to an aromatic ring

aromatic: Hs bonded to a C in a benzene ring

aliphatic: H bonded to sp3 C bonded to another sp3 C

(most reactive) benzylic > aliphatic >~ aromatic

describe the C-H bonds that react with bromine for toulene, ethyl benzene, isopropyl benzene, and tertbutyl benzene

toulene: primary, benzylic

ethyl benzene: secondary benzylic

isopropyl benzene: tertiary benzylic

tertbutyl benzene: primary aliphatic

what is the general reaction for the arenes lab?

R-H + Br2 + hv --> R-Br + Hbr

what are the three general steps for the arenes rxn? **be able to draw out the complete reaction**

(1) initiation- formation of Br radicals

(2) propogation

(3) termination

describe the first 6 steps of the arenes procedure (prior to start of rxn)

1. obtain 8 disposable test tubes and a beaker to hold the tubes

2. label 4 tubes with the # corresponding to the compound

3. add 2 mL DCM to each numbered test tube

4. add 0.5mL of each substrate to it's corresponding numbered tube

5. add 0.5mL of bromine soln to the 4 remaining tubes.

6. place all 8 tubes in an ice-water bath for 5 min

describe the last 4 steps of the arenes procedure

6. add one of the bromine tubes to 1 of the substrate tubes and and record the time (seconds) when each tube becomes colorless and in what order (start timing @ addition of bromine)

7. once the order has been determined for all test tubes, pour all into an erlenmeyer flask

8. quench residual bromine by adding 3-4 drops of cyclohexene to the flask or enough so the soln is colorless

9. dispose soln in halogenated organic waste

what two compounds are very reactive with bromine and what should be done about it?

toulene and ethyl benzene

add the bromine to these last, they may change color before you finish with the other samples

have one partner time while the other adds the bromine to the test tubes

what should the test tubes be rinsed with before disposing them in the glass waste?

DCM

what should you do if the bromine does not mix in with the compound/soln?

bromine is not always immediately miscible

use a pipette and bubble air from the bottom to mix the solns

what is in excess for this experiment?

hydrocarbon arene substrate (limiting reagent is the bromine)

how should you clean up a bromine spill?

pour sodium thiosulfate around the spill. Once the red color disappears, the mess can be treated as halogenated organic waste or wiped up with paper towels and discarded in the trash

what are 3 ways to properly use cyclohexene?

(1) don't waste cyclohexen! It smells terrible, the less you use the better

(2) you just need 1 drop for every test tube still red. (3) Solutions should be bubbled with a pipette to mix in the cyclohexene

where should you get/not get chemicals to clean glassware?

NOT from reagent hood

FROM solvent hood

what should you not wash glassware that contained bromine with?

acetone

what is substitution?

reactions where one molecule is replaced by another atom/group of atoms

what is free radical substitution?

involves breaking a C-H bond in alkanes and forming a new bond to something else

what is a radical?

a species with an unpaired election due to homolytic cleavage

what is a photochemical rxn?

reaction brought about by light (energy)

what does statistical product distribution mean?

every H on the most stable C has an equal likelihood of being replaced by bromine

does radical bromination substitution often stop at a single substitution?

no

why is Br2 more selective than Cl2?

bromination is slower and more selective because the bromine atom is less reactive than the chlorine because Br is less electronegative than Cl and wants e-s less

how do you know the arene reaction is completed?

the red color will go away

why are we pouring all test tubes into a flask before pouring them into liquid waste?

to decrease the chances of spilling and dropping the test tubes

what data are we collecting for the arenes lab?

time in seconds when the color changes from red to clear

what is the rxn type for the bromination of E-stilbene?

an addition mechanism

alkene halogenation

how many products can be produced when E-stilbene undergoes bromination, and what are they? **draw these on paper**

3

meso-stilbene dibromide, l-stilbene dibromide, d-stilbene dibromide

what is the isomeric relationship btwn the meso, d, and l isomers?

meso is an diastereomer to the d and l

d and l are enantiomers

what are diastereomers?

stereoisomers that are not mirror images

what are enantiomers?

non-superimposable mirror images

what does the distribution of the isomers depend on?

the cation intermediate of the reaction

what are the two cation types and what are the isomeric distributions?

cyclic bromonium cation: 100% meso

acyclic carbocation: 50% meso, 50% d&l (25%d, 25%l)

what is the actual distribution of isomerization? why?

neither, it is around 90% meso, 10% d&l (5%d, 5%l)

both cation intermediates are formed during the reaction (the cyclic bromonium cation is dominant)

what are we isolating in the e-stilbene experiment?

the meso-stilbene dibromide

what is our expected yield and what is a good yield?

50% expected

70% is good

what data are we collecting in the e-stilbene experiment?

MP and % yield

what is the likely cause of a low % yeild?

poor vessel transfer

how do we set up the glassware for the e-stilbene experiment? (3 things)

1. lightly cap the round bottom flask by resting the glass stopper on top of the keck clamp

2. make sure the reaction is not sealed to avoid pressure buildup; air should be able to flow around the stopper

3. it is recommended to clamp the round-bottom flask in place

describe the 6 steps of the stilbene procedure

1. in a 25mL round-bottom flask, add 0.9g of E-stilbene and 10mL DCM and a 1 inch stir bar

2. stir to dissolve the solid, then add 5mL of 1M bromine soln

3. let it stir for 10-15 min, until most of the orange color has disappeared and a white solid has formed

4. filter the precipitate using a buchner funnel and wash w/ ~2mL of cold DCM to ensure the removal of most of the bromine

5. obtain the MP and % yield

6. save the product for dehydrobromination

what does the color do over time

the orange color disappears and the soln becomes colorless

is the bromination of an alkene an oxidation or reduction?

oxidation (alkene --> alkene = less e- density)

how do you use the mel-temp?

carefully load about 3mm of sample per tube into capillary tubes. Watch through the eye piece as temp increases. The rate of T should be no more than 2*C per minute. Use known melting pts to gauge the appropriate starting T

how quickly should T be increasing on the mel-temp?

no more than 2*C per minute

are halogen cations stable (like the Br cation in the bridged bromonium intermediate)?

no they are unstable, unhappy, and very reactive

when a bridged bromonium intermediate is formed, describe the stereochemistry **be able to draw the mechanisms of the rxns to form each isomer**

the reaction is an inversion due to a backside attack because the bridge bonds hinder/block the halide's electrons (Br-) from reaching the positively charged carbon atom

what are the rxn conditions for the alkene halogenation required to avoid unexpected products?

an inert solvent

how does the alkene react with the Br2?

the neutrophilic e-s in the pi bond of the alkene are strong enough to induce a temporary polarity on the neutral halogen (Br-Br) to grab the temporarily partial positive halogen (the electrophile)

how do you calculate the limiting reagent?

1. calculate the # of moles of each by multiplying the V x Molarity.

2. divide the # of moles of each by its stoichiometric coefficient in the balanced chemical equation

what is the general equation for the bromination of arenes lab?

R-H + Br2 ---(heat/hv)--------> R-Br + H-Br

what is the starting reaction for the arene lab?

initiation step (splitting of Br2 molecule w light), but propogation is when the arene molec first reacts

aliphatic carbons? (textbook definition)

hydrogens bonded to sp3 hybridized carbons (can be primary, secondary or tertiary)

allylic/benzylic carbon? (textbook definition)

aliphatic H attached to sp3 C that also bound to a vinylic/aromatic C

vinylic/aromatic C?

Hs attached to sp2 Cs

acetylenic carbon?

sp hypridized C attached

time for arene reaction to react depends on ______?

the reaction's conditions & relative reactivities of the various hydrocarbons

what results in a free radical?

homolytic fission! the splitting of a bond evenly so that each atom gets one of the 2 electrons.

what should you add into the erlenmeyer flask in addition to the 0.9g of stilbene?

10 mL dichloromethane w sitrbar

experimental goal of e-stilbene lab?

brominate E-stilbene & isolate meso product

why do we not get perfect 50/50 (meso/d&l) or 100% meso as the carbocations predict?

maybe bc we get both the acyclic carbocation & cyclic bromonium instead of one of them (textbook)

what are the 2 possible cations that can react w the e-stilbene rxn?

acyclic carbocation & cyclic bromonium

which product is expected from the acyclic carbocation (stilbene lab)?

50/50 of meso/d&l

which product is expected from the cyclic bromonium (stilbene lab)?

100% meso