Alkenes Flashcards

1
Q

what are alkenes

A

unsaturated hydrocarbons that contain a C=C double bond

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2
Q

describe the bonding in alkenes

A
  • the C=C consists of a σ bond - the direct overlap of s orbitals between bonding atoms
  • also contains a π bond - the sideways overlap of of adjacent p-orbitals above and below the plane of the molecules
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3
Q

why are pi bonds much weaker than sigma bonds

A
  • e- density is delocalised above and below plane of molecule
  • this means that the electrostatic attraction between the nuclei and a shared pair of e- is weaker, so pi bonds have relatively low bond enthalpy
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4
Q

why do sigma bonds (strongest type of covalent bond) have high bond enthalpy

A
  • high e- density between the nuclei means there is a strong electrostatic attraction between the nuclei of the bonded atoms and shared pair of e-
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5
Q

what shape does the C of the double bond have, and what’s the angle

A
  • trigonal planar
  • 120 degrees
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6
Q

why does C in the double bond have its shape

A
  • 3 areas of e- density (bonding pairs) around C=C atoms repel equally
  • 0 lone pairs
  • lone pairs repel more than bonding pairs
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7
Q

why are alkenes relatively reactive

A
  • carbon carbon double bond has high electron density and so pi bond can be attacked by an electrophile
  • π bond has low bond enthalpy
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8
Q

why can alkenes exhibit stereoisomerism

A

C=C restricts rotation

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9
Q

define stereoisomerism

A

compounds with the same molecular and structural formula, but a different arrangement of atoms in space

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10
Q

what’s E-Z isomerism

A

stereoisomerism that arises due to the restricted rotation about the C=C bond when there are 2 different groups attached to each carbon of the C=C

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11
Q

how to use E-Z system

A
  1. assign priority to each group attached to the first C (atom with the highest atomic number)
  2. assign priority to each group attached to second C
  3. two priority groups on same side of double bond = Z isomer (zame zide)
    opposite sides = E isomer
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12
Q

what’s cis-trans isomerism

A

special case of E-Z isomerism where 2 groups attached to each C of the C=C bond are the same

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13
Q

how can alkenes readily undergo addition reactions

A
  • π bond e- can be used to form new bonds with an attacking molecule
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14
Q

general equation for hydrogenation and condition

A

alkene + H2 –(Ni)–> alkane

  • reactants heated
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15
Q

what can hydrogenation be used for

A
  • in manufacture of margarine from vegetable oils
  • vegetable oils are unsaturated and contain C=C
  • hydrogenation converts these into C-C and changes the physical properties, so oil converted into solid margarine
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16
Q

define electrophile

A

e- pair acceptors attracted to areas of high e- density

17
Q

what’s the reaction of alkenes with electrophiles

A
  • electrophilic addition
  • electrophiles are attracted to e- in C=C
18
Q

general equation for the bromination of alkenes

A

alkene + bromine —-> dibromoalkane

19
Q

how does bromination of alkenes occur

A

alkene polarises bromine molecule, which then acts as an electrophile and adds to the alkene

20
Q

what’s bromination of alkenes used to test for

A
  • unsaturation
  • bromine water will go from orange to colourless if Br is added to alkene
21
Q

how does the Br-Br bond break

A
  • via heterolytic fission
  • both bonding e- go to same Br atom, forming Br- ion
22
Q

general equation for addition reaction of alkenes with steam

A
  • alkene + H2O (g) <–(H3PO4)–> alcohol
    conditions: 300C, 60 atm
  • reversible
23
Q

addition reaction alkenes with steam is also an example

A

hydration reactions

24
Q

with any unsymmetrical molecule there is possibility of forming …. products in addition reactions

25
what's type of reaction occurs between hydrogen halides and alkenes and what does it form
- addition - forms haloalkanes
26
define carbocation
molecule that contains a C atom with a +ve charge
27
what makes a carbocation more stable and why
- more **alkyl groups** (C) attached to +vely charged C - alkyl groups are **elctron-releasing** so they help spread the charge
28
what's Markownikoff's rule
- major product is formed via the most substituted carbocation intermediate - 1* carbocations are the least stable, 3* are most stable
29
what are addition polymers (of alkenes)
- saturated chains formed from alkene monomers - alkene double bonds open and form the long chain polymers
30
what are the 4 methods that scientists have developed to make polymer use more sustainable
- recycling - incineration - organic feedstock - bioplastics: biodegradable and photodegradable polymers
31
description, benefits and issues of recycling polymers
- polymer waste tediously and inefficiently sorted before being processed and used to make new materials - conserves natural resources - some polymers like PVC, difficult to recycle as toxic and corrosive additives first need to be removed
32
description, benefits and issues of incineration
- polymer waste combusted to convert it into heat energy for use in electricity generation - produces toxic waste gases (HCl) which can be removed from incinerator chimneys
33
description, benefits and issues of organic feedstock
- polymer waste broken down into gases, mainly H2 and CO or oil - these can be used in other chemical reaction like cracking, or as raw materials for new plastics and other chemicals
34
description of biodegradable polymers
- plant based, broken down by microbes
35
description of photodegradable polymers
- oil based, but broken down by absorbing light