Halogenation & Alkenes

krabi's version from 2015-06-09 00:19

Section 1

Question Answer
What is Halogenation? Alkane + halogen ⇒ in presence of heat/UV light to create a free radical.
How does halogen take place?The energy from heat/light homolytically cleaves diatomic halogen
What happens during homolytic cleavage?Each atom in the bond has a goody bag: one e- from the broken bond
What is the result of halogenation? 2 highly reactive species each ⇒ with an unpaired electron & a free radical (which are the active reactive species).
What happens during initiation in halogenation? CLEAVE: Halogen is a diatomic molecule ⇒ molecule is homolytically cleaved by heat/light ⇒ resulting in free radicals
What happens during propagation in halogenation?PLUCK: The halogen radical plucks an H ⇒ leads to alkyl radical. This opens it to react w/atomic halogen molecule.
Can propagation continue forever? YES -- it can be the neverending story OR go to termination.
What happens during termination in halogenation?Two radicals bond or a radical bonds to the wall of container to terminate.
What is the energy process of halogenation? Is exothermic
What is the stability of halogenation? Same as carbocation stability ( 3 > 2>1>methyl)

Section 2

Question Answer
Why are alkenes more reliable?They have Pi bonds which are more reactive and less stable; Remember Pi bonds are electron hungry (Pie eaters are hungry for e-)
Why are alkenes acidic?If a proton is plucked off; THEN, the Pi bond of alkene absorbs the negativity to stabilize the conjugate base.
Is a Pi bond neg or positive?It is a large area of negativity & therefore, attractive to electrophiles.
When are alkenes most stable? When they are the most substituted with R’s ⇒ when they are surrounded by their friends
When are alkenes UNstable?When they are least substituted, and are attached to H’s ⇒ when they do not have their friends
What are the physical property trends of alkenes?Increase in molecular weight ⇒ ++ BP Branching decreases BP.
Compare the physical properties of alkenes and alkanes:Alkenes are more acidic than alkanes Alkenes have lower density than water, and are slightly soluble in water (unlike alkanes).
Compare the physical properties of alkynes and alkenesAlkynes are more polar than alkenes ⇒ and more soluble

Section 3

Question Answer
How are alkenes joined? Through elimination rx: 1-2 functional groups are eliminated ⇒ to create a double bond.
Give an examples of an E1 rx? Dehydration of alcohol - booze forms in presence of hot acid. The rate depends on the concentration of alcohol.
What is the first step of E1 rx?PROTONATE: The acid protonates the OH group ⇒ produces a Good Leaving Group GLG
What is the 2nd step of E1 rx?REARRANGE: THe water drops off and forms a CC. Rearrangement happens post-CC formation.
When does CC rearrangement happen?Only if a more stable CC can be formed
What is the rate determining step during an E1? It is the 2nd step, slowest step of water loss.
What is protonation?Protonation is the addition of a proton (H+) to an atom, molecule, or ion ⇒ forming the conjugate acid.
What is a conjugate base?The leftovers ⇒ conjugate base of an acid is formed when the acid already done its work donates a proton.
What is a conjugate acid?A conjugate acid, within the Brønsted–Lowry acid–base theory, is a species formed by the reception of a proton (H+), by abase—in other words, the base with a hydrogen ion added to it.