Ch. 10 - Alkenes & Addition Reactions (Week 2, Quiz 2) Flashcards
What are alkenes?
compounds that contain C-C double bonds
What are terminal alkenes?
have a double bond at the end of the carbon chain
What are internal alkene?
have a least one C atom bonded to each end of double bond
What are cycloalkenes?
have double bond in the ring
What makes up a double bond?
one pi bond and one sigma bond
What is hybridization, geometry and angles of pi bond carbons?
sp2, trig planar, 120º
Which is weaker: sigma or pi bond?
pi bond is weaker, easier to break so alkenes go under many reactions
Why do pi bonds go under many reactions?
have pi bond that is easy to break
T/F: Double bonds have free rotation.
False, double bonds have restricted rotation. For rotation to occur, double bond would have to break
What is stereoisomerism of double bond?
when 2 groups on each C=C end are different, two diasteriomers are possible
What are the two diastereomers possible for a carbon double bond with stereoisomerism? Which is mores stable?
cis and trans; trans is more stable
What is the formula for an acyclic alkene?
CnH2n
Why are alkenes considered unsaturated?
they have less than the max number of hydrogens per carbon
What is the max number of hydrogens a carbon chain can have?
2n+2
How are alkenes named?
1) name longest C chain with both carbons of double bond and change -ane to -ene
2) number the chain giving the double bond the lower number
What are dienes?
two double bonds
What are trienes?
three double bonds
What are alkenols?
have both a double bond and a hydroxyl group
ex. prop-2-en-1-ol
How are cycloalkenes named?
give double bond carbons numbers 1 and 2 and then give lowest number to subs
What types of IMFs do most alkenes have?
just weak van der Waals
What are MP/BP and solubility properties of alkenes?
low MP/BP that increases and number of carbons/surface area increases
soluble in organic solvents but not in H2O
Why do cis and trans alkenes have slightly different properties?
sp3 hybridized alkyl carbon donates electron density to sp2 hybridized alkenol carbon which creates a slight polar bond
in a cis isomer, dipole moments are reinforced so there is a net dipole
in trans isomer, dipole moments cancel so there is no net dipole
What are the three ways so far that alkenes can be prepared?
1) dehydrohalogenation of alkyl halides with a strong base (E2)
2) acid-catalyzed dehydration of alcohols with H2SO4 or TsOH (E1/E2)
3) dehydration of alcohols with POCl3 and pyridine (E2)
What occurs in a general addition reaction?
a pi bond breaks and 2 new sigma bonds form
T/F: alkenes are electron poor
false, they are electron rich
What do simple alkenes react with (general)?
electrophiles
What are SM and P of hydrohalogenation?
SM = alkene P = alkyl halide
What are reactants of hydrohalogenation?
H-X (X = Cl, Br, I)
What is the mechanism of hydrohalogenation?
addition (Sn1)
What are the major steps of alkene hydrohalogenation?
an alkene double bond electrons bind to H and HX bond breaks giving electrons to X (forms carbocation)
nucleophilic X- attacks carbocation via Sn1 to make alkyl halide
Which is the RDS and fast steps of hydrohalogenation? Aka which step has the highest activation energy?
RDS = formation of carbocation (breaking pi bond) *has higher activation energy* fast = Sn1 to make alkyl halide
Which hydrohalogenation transition state has highest energy?
state between alkene and carbocation
What could the two reactions for hydrohalogenation be classified as in terms of acids/bases?
Lewis A-B reaction
Why are addition reactions exothermic?
sigma bonds formed are stronger than the pi bonds broken
What is Markovnikov’s rule?
in addition of HX to unsymmetrical alkene, the H atoms binds to the LESS sub’d carbon to give a MORE sub’d carbocation (the most stable carbocation will form)
What configurations will be given when an electrophile attacks a carbocation and the carbon becomes a stereocenter?
both configurations will be giving (R and S); will form a racemic mixture
Does Markovnikov’s rule see the future?
No, so carbocations can’t know if they will be able to undergo a methyl or hydride shift, therefore, even if one can and the other can’t, they will both form because molecules can see the future!
Can Mark’s rule see resonance?
Yes! A carbocation wants to be closer to double bonds so it can be resonantly stabilized. So the carbocation with resonance stabilization will form and the other won’t.
What explains Marl’s rule? (another rule)
Hammond’s Postulate
- for an endothermic reaction, the transition state is closer in structure and energy to the product
- what stabilizes the product will stabilize the transition state
What is the name for a mixture of diastereomers?
scalemic
If a molecule that already has a stereocenter undergoes alkene addition with HX and the alpha carbon will become a stereocenter, what will the products be?
diastereomers!
For naming alkenes, are the numbers of both carbons of the alkene added to the name?
no, only the first carbon (lower number)
Why do enantiomers form for alkene hydrohalogenation (HX addition)?
because addition occurs from top and bottom of planar double bond
Do syn or anti products form for alkene + HX = alkyl halide?
both syn and anti
What makes HX addition regioselective?
Mark’s rule (H bonds to less sub’d carbon to give more sub’d carbocation)
What is the RDS for HX addition?
formation of carbocation
What are SM and P of acid-catalyzed alkene hydration?
SM = alkene P = alcohol
What are reagents for acid-catalyzed alkene hydration?
water and acid (H2SO4 or TsOH)
What is mechanism for acid-catalyzed alkene hydration?
Addition, Sn1
What are major steps of acid-catalyzed alkene hydration?
alkene is protonated by acid (H goes to less sub’s carbon) … Mark’s rule… forms carbocation
nuc attack by water on carbocation via Sn2 forms more sub’d alcohol
What is the solvent of acid-catalyzed alkene hydration?
water
Can RAR occur for acid-catalyzed alkene hydration?
Yes, proceeds by Sn1 so carbocation
Can alkene reactions occur on aromatic rings?
No ma’am
What type of products (stereochemistry) does acid-catalyzed alkene hydration form?
if st, center results, then racemic mixture
What type of addition products (syn or anti) does acid-catalyzed alkene hydration form?
both syn and anti
Why is acid-catalyzed alkene hydration considered “acid-catalyzed/”
because reaction regenerates acid
What are three consequences of carbocation intermediates?
1) H adds to less sub’d carbon to form more stable carbocation (mark)
2) addition occurs in syn + anti
3) RAR can occur
How many times does water react in acid-catalyzed alkene hydration?
twice
What are SM and P of acid-catalyzed alkene hydroalkoxylation?
SM = alkene P = ether
What are reagents of acid-catalyzed alkene hydroalkoxylation?
acid (H2SO4 or TsOH) and ROH
What is mechanism of acid-catalyzed alkene hydroalkoxylation?
Sn1
What are major steps of acid-catalyzed alkene hydroalkoxylation?
alkene is protonated with acid and carbocation forms following Mark’s rule
nuc attack by ROH oxygen on carbocation gives alcohol via Sn1
What is the solvent of acid-catalyzed alkene hydroalkoxylation?
alcohol
What type of products are formed (stereochemistry) by acid-catalyzed alkene hydroalkoxylation?
racemic
Does syn or anti occur for acid-catalyzed alkene hydroalkoxylation?
both syn and anti products (racemic mixture of each)
What makes acid-catalyzed alkene hydroalkoxylation regioselective?
follows Mark’s rule
Can RAR occur for acid-catalyzed alkene hydroalkoxylation?
Yes, Sn1
What reactions is acid-catalyzed alkene hydroalkoxylation exactly like?
acid-catalyzed alkene hydration (making an alcohol from an alkene)
What causes X2 to be momentarily polar?
momentary dipole is formed when electron cloud of Br2 oscillates
What are SM and P of alkene dihalogenation?
SM = alkene P = vicinal dihalide
What are reagents of alkene dihalogenation?
X2 where X = Cl or Br only
Why can’t F or I react in alkene dihalogenation?
F is too reactive and I is too reversible
What is mechanism for alkene dihalogenation?
anti addition, Sn2
What are major steps for alkene dihalogenation?
pi bond electrons go to Br and same Br bonds to other side of pi bond to make bridged halonium ion
other Br- attackes MORE sub’d carbon (the carbon that can better handle partial positive charge) and then becomes vicinal halide via Sn2
Can alkene dihalogenation undergo RAR?
No, Sn2 so no carbocation forms
What type of addition is X2?
anti only
What are stereochemical considerations of alkene dihalogenation?
anti product is made exclusively
two bridged halonium ions form (one with dashes and one with wedges) so racemic products form after backside attack
cis and trans alkene isomers each give racemic mixture which are diastereomers of eachother
Why is alkene dihalogenation considered stereospecific?
one stereoisomer forms (anti not syn) note: still racemic!
What is the difference between stereoselective and stereospecifc?
stereoselective is when one stereoisomer forms (anti or syn) and stereospecific is when two or more stereoisomers form but one is major/minor
Why is the more sub’d carbon attacked on the holonium ion of alkene dihalogenation?
because it better holds positive charge
What are SM and P of alkene halohydration?
SM = alkene P = halohydrin
What is a halohydrin?
has an alcohol group and X group on adjacent carbons
What are reagents of alkene halohydration?
X2 (X=Cl,Br,I) and water
What is mechanism of alkene halohydration?
anti addition, Sn2
What are major steps of alkene halohydration?
an alkene reacts with X2 to form bridged halonium ion (x2)
Water attacks the carbon where partial charge would be most stable to open ring
Water depronates H2O to make alcohol group
Does alkene halohydration undergo RAR?
No, Sn2 so no carbocation
What is the solvent of alkene halohydration?
water
What type of addition is alkene halohydration?
anti addition only
What are the stereochemical considerations for alkene halohydration?
always anti addition
2 bridged halonium ions form so both anti trans products form
cis and trans alkenes make racemic mixtures that are diastereomeric of eachother
Why does water act as nuc and not X- in alkene halohydration?
Because water is in much greater concentration (1000000:1)
With what other reagents can alkene halohydration occur?
NBS (just a source of Br2)
DMSOm H2O
Why is alkene halohydration regioslective?
water attacks the more sub’s carbon to deal with partial positive charge
What are SM and P of brown alkene hydroboration-oxidation?
SM = alkene P = anti-mark. alcohol
What are reagents of brown alkene hydroboration-oxidation??
BH3 and H2O2, NaOH
Where does alcohol end up for brown alkene hydroboration-oxidation?
on the LESS sub’d carbon
Are there carbocations in brown alkene hydroboration-oxidation?
No
Why is borane electrophilic?
it has an empty p-orbital
Where does BH2 end up in hydroboration?
less sub’s carbon (like alcohol)
What kind of addition is brown alkene hydroboration-oxidation?
always syn addition, forms racemic mixture
What am I going to tell you to do?
Stay calm, have fun, enjoy the moment and be proud
