Ib Chemistry Iii Organic: Oct. 28 Test

Chem Review: Organic Reaction Pathways, Mechanisms, and Free Radicals Test Tues. Oct. 28, 2008 Theory By Brittanie; Reactions by Autumn This is the nitpicky section, or theory, as some people like to call it, whatever, we all hate it dearly. At least we can agree upon that! Rxn Pathways: lets start with the gigantic task and work our way down, agreed? OK, good. Alkene  poly alkene ; break the double bond(essential for addition polymerization) and add The ALKANE version of the alkene to itself, remember to mark off the monomer(the one thing that repeats). There is also an organic peroxide, But we ever seem to put it, but anyway it is OR with one of those dots for radicals infront of it. Alkene  Dihalogenoalkane ; reacts with a halogen (usually CL2 or Br2) and CCl4 is the Catalyst, break double bond and place halogens on carbons that once had double bond To a multihalogenoalkane; reacts with the halogen again and UV light is needed Alkene  Alkane; rxts with H2 and platinum is the catalyst, break double bond, and put H Where double bond was. , if you want to add extra Lovely knowledge… BROMINE TEST: if you react Br with an alkene, then the color will Change from orange to colorless (because the Br is all used up  poor Br) Alkane Halogenoalkane; alkane rxts with halogen in UV light Halogenoalkane  alcohol is a Sn2 mechanism w/ OH- and this rxn must take place in Warm environment (You can’t bake a cake on the kitchen counter, Hillary!)

If the alcohol is PRIMARY, then you can DISTILL it with a catalyst of K2Cr2O7 in a acidic solution to produce an ALDEHYDE! Or you can REFLUX it with the same catalyst and solution requirements to produce a CARBOXYLIC ACID!!! If the Alcohol is SECONDARY, then you can react it with the same catalyst and solution as above and produce a KETONE!!! NOTE: In the above reactions the Cr2O7-2 ion is ORANGE, when oxidize(as above) it Changes to the Cr+3 which is BLUE! If the alcohol is TERTIARY, you cannot oxidize it, the only way you would get a double bonded Oxygen is to break a C-C bond, which would be bad and yucky, so we don’t do it!!!! Just to make sure: TERTIARY alcohols CANNOT be oxidized like the SECONDARY OR PRIMARY alcohols, ok? Are we good? Are you sure? You might want to read it one more time, just to cover your tracks? Did you read it again? Good, you rock....seriously. Theory from the Yellow Sheet Titled Topic 10&20 1. The OH- is a charged particle, so it more readily donates the e- than a water molecule. 2. The less electronegative the halogen, the more easily the halogen leaves the halogenoalkane, thus making the rxn faster. Halogens in order of best leaving group: I > Br > Cl > F 3. Tertiary halogenoalkanes have steric hindrance, which means it has to be Sn1, which are the fast rxns, because the nucleophile cannot attack from 180 degrees of leaving group. Primary halogenoalkanes allow for attacks 180 degrees from the leaving group, thus they react with a Sn2 mechanism, which is the slow mechanism. Secondary Halogenoalkanes can react as either Sn1 or Sn2, with Sn1 still being the faster of the two. 4. Alkenes produce industrial alcohols, like Ethanol. Ex: Alkene reacts it with water with H2SO4 as a catalyst and produces the alcohol. End yellow sheet

Order of best nucleophiles: leaving groups: CNOHNH3 H2O

Order of Best I Br Cl F

If there is a question on benxene rings, which I doubt there will be, it will probably want you to explain this: If the leaving group is attached directly to the ring, then it acts as a tertiary halogenoalkane in which a Sn2 rxn cannot take place (there is no place 180 degrees from the leaving group to put the nucleophile!!!) However, if CH2-Cl is attached to a Benzene ring by the C of that group, then it can be Sn2 because of the spot 180 degrees away from the Cl on that attaching carbon, Yay!!! Sn2 gets a break. You know I always have thought of the Sn2 mechanism as being the Trix rabbit. It is almost like people go “Oh, Silly Sn2, Tertiary halogenoalkanes are for Sn1 mechanisms!” Ok, so maybe it’s not as catchy as the original slogan, but it gives me a giggle (chuckle if you’re a guy reading this). Racemic mixtures! Sn1 is almost always racemic mixtures, fyi. Sn2 are racemic mixtures when 50% of the reactants have been reacted, but they do not angle the light!!!!! They are still optically active though. So, if I have anyway confused you or if you would prefer a more “serious” study guide please visit your little black book (pgs 228-236) or our green IB book(pgs. 64,66,and 71). Good luck everybody, u guys rock!!!!!!!!!!!

Reactions 1. Homolytic Vs. Heterolytic Homolytic is the free-radical one Think of it as a “ho” having free, radical love: Ho-Fusion A∙ + B∙  A:B

Ho-fission

A:B  A∙ + B∙

Heterolytic is where one reactant, or “species,” gets all the charges: Het-Fusion: A- + B+  A:B Het-fission: A:B  A- + B+ 2. SN2 Mecanisms (You should have SN2 in your notes before SN1, so I’m following the trend. It messed me up yesterday on homework, so remember: the notes are always counting down. 2, then 1.) Single-Step, produces Intermediate (that annoying thing in brackets):

Notice that the OH group (reversed to show that the oxygen is really the important thing) is sneaking up on the carbon from the opposite side from the Br. That’s what SN2 reactions do: they give twotiming (get it? 2-timing!! Ha ha) OH groups the chance to grab those poor primary carbons from behind. If the carbons were surrounded with a bunch of buddies, then the little SN2’s couldn’t do that: only a big, tough, “number one” SN1 can (get it? Number 1!! Hee hee)… which brings us to: 3. SN1 Mechanisms! These are the fast, strong, two-step processes that beat up even carbons that travel surrounded by lots of themselves (ie, tertiary):

1.

SLOW step

2.

FAST step

4. Finally, Free Radicals (everyone’s favorites…) The reaction itself is: Alkane (eg. Ch4) + Cl2 Halogenoalkane (eg. CH3Cl) + HCl The Mechanism is in Three steps: 1. Initiating Cl2

Cl∙ + Cl∙

2. Propagating (2 subs)

And

CH4 + Cl∙  CH3∙ + HCl CH3∙ + Cl2  CH3Cl + Cl∙

(Remember that the two propagating steps eventually end in a Cl-free-rad) 3. Terminating (3 subs) Cl∙ + Cl∙

Cl2

Or

C2H5. ∙

C2H5.:C2H5

Or

Cl∙ + C2H5. ∙

C2H5Cl

NOTICE THE LONE ELECTRONS ON ALL OF THESE REACTIONS!! …and you’ll do fine. There are no addition, halogenation, etc., reaction diagrams on this guide. The theory hopefully explains it well enough. For any other questions, please consult your green guidebook, pages 64-67, 69-70, and the top of 71.

Thank you and good night! Love, Autumnah the editor.