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spirit burner (chemistry) > Chapter 13: Alkenes > Flashcards

Chapter 13: Alkenes Flashcards

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1
Q

What are alkenes?

A
  • unsaturated hydrocarbons

- contain at least one carbon to carbon double bond in their structure

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2
Q

Alkenes: The nature of the double bond

A
  • For each carbon atom of the bounce bond, three of the four electrons are used in three sigma bonds, one to the other carbon atom of the double bond and the other two electrons to two other atoms
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3
Q

Alkenes: The nature of the double bond (continued)

A
  • This leaves one electron on each carbon atom of the double bond not involved in sigma bonds
  • This electron is in a p orbital
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4
Q

When is a pi bond formed?

A

Sideways overlap of two p orbitals, one from each carbon atom of the double bond

  • Each carbon atom contributes one electron to the electron pair in the pi bond
  • the pi electron density is concentrated above and below the line joining the nuclei or the bonding atoms
  • The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond
  • This makes the geometry of the alkenes different to that of the alkanes where rotation is possible around every atom
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5
Q

The shape around a double bond

A

It is trigonal planar because:

  • There are 3 regions of electron density around each of the carbon atoms
  • The three regions repel each other as far apart as possible, so the bond angle around each carbon atom 120 degrees
  • All of the atoms are in the same plane
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6
Q

What are stereo-isomers?

A
  • They have the same structural formula but a different arrangement in space
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7
Q

Why does stereo-isomerism around the double bond arise?

A
  • Rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed to each other
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8
Q

What is the reason for the rigidity around the double bond?

A
  • The position of the Pi bonds electron density above and below the plane and the sigma bond
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9
Q

What conditions do molecules have to satisfy in order to display E/Z isomerism?

A
  • A C=C double bond

- different groups attached to each carbon atom of the double bond

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10
Q

What is cis-trans isomerism?

A
  • A name commonly used to describe a speacial case of E/Z isomerism
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11
Q

What is one of the main conditions of cis-trans isomerism?

A
  • One of the attached groups on each carbon atom of the double bond must be hydrogen
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12
Q

Characteristics of the cis isomer:

Characteristics of the trans isomer:

A
  • The cis isomer has the hydrogen atoms on each carbon in the double bond on the same side of the molecule
  • The trans isomer has the hydrogen atoms diagonally opposite each othe r
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13
Q

Using the Cahn-Ingold-Pregold rules

A

In this system the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number

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14
Q

If the groups of the higher priority are on the same side of the double bond

A

is the Z isomer

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15
Q

If the groups of higher priority are diagonally placed across the double bond

A

The compound is the E isomer

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16
Q

How do you assign priority

A
  • Examine the atoms attached directly to the carbon atom of the double bond and decide which of the two atoms has the highest priority
  • The higher the atomic number, the higher the priority
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17
Q

Why are alkenes more reactive than alkanes

A

Because the Pi bonds

  • Being on the outside of the double bond, the Pi electrons are more exposed than the electrons in the sigma bond
  • A Pi bond readily breaks and alkenes undergo addition reactions relatively easily
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18
Q

Which bond is weaker, the Pi or sigma bond?

A

The Pi bond

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19
Q

What is an addition reaction?

A
  • The addition of a small molecule across the double bond, causing the Pi bond to break and for new bonds to form
20
Q

Alkenes undergo addition reactions with hydrogen halides to produce haloalkanes.

What reagents are needed?
What conditions are needed?

A
  • The hydrogen halide e.g. HBr

* Normal conditions

21
Q

Alkenes undergo addition reactions with water to produce alcohols. (hydration)

What reagents are needed?
What conditions are needed?

A
  • Phosphoric acid catalyst
  • 300 °C
  • 60atm pressure
22
Q

Alkenes undergo reactions with hydrogen to produce alkanes.
What reagents are needed?
What conditions are needed?

A
  • Nickel catalyst
    *150°C
  • 5 atm
    or
    *Platinum catalyst
  • Normal conditions
23
Q

How would you test for unsaturation?

A

1) Add bromine water (an orange solution)
2) Observe the colour change (If it changes from orange to colourless)
3) Indicating the presence of a C=C double bond

24
Q

What is electrophilic addition?

A
  • Alkenes take part in addition reactions to produce saturated products
25
Q

Why does the double bond in an alkene represent a region of high electron density?

A
  • The presence of the sigma electrons
26
Q

What does the high electron density attract?

A

electrophiles

27
Q

What is an electrophile?

A
  • An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
  • An electrophile is usually a positive ion or molecule containing an atom with a partial positive charge
28
Q

The mechanism for the electrophilic addition reaction of but-2-ene with hydrogen brominde

A
  1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar (Hydrogen is partially positive and Bromine is partially negative)
  2. The electron pair in the Pi bond is attracted to to the partially positive hydrogen atom, causing the double bond to break
  3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon aton that was part of the double bond
  4. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
  5. A bromine ion and a carbonation are formed. A carbocation contains a positively charged carbon atom
  6. In the final step the Br ion reacts with the carbocation to form the addition product
29
Q

Markownikoff’s rule

A

Markowninokoff’s rule states when a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms

30
Q

Carbocation stability: Why are tertiary carbocations most stable

A
  • Each alkyl group donates and pushes towards electrons towards the positive charge of the carbocation
  • The positive charge is spread over the alkyl groups
  • The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out, making the ion more stable
  • Therefore tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations
31
Q

What was the first synthetic polymer called?

What was it used as?

A
  • Bakelite

- An electrical insulator

32
Q

What is a polymer?

A
  • Large molecules formed by many repeating units of smaller molecules known as monomers
33
Q

Characteristic of an addition polymer

A
  • High molecular mass
34
Q

Characteristics of synthetic polymer

A
  • Usually named after the monomer that reacts to form their giant molecules, prefixed by ‘poly’
35
Q

A general rule for repeating units:

A
  • A repeating unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again
  • The repeat unit is always written in square brackets
  • After the bracket you place a letter n to show that there is a large number of repeats
36
Q

Poly(chloroethene)

A
  • also known as PVC or poly(vinyl chloride)

can be prepared to make a polymer that is flexible or rigid

37
Q

Environmental concerns:

Disposing of waste polymers

A
  • The lack of reactivity makes polymers suitable for storing food and chemicals safely
  • Many alkene based polymers are non-biodegradable
  • Serious environmental effects e.g. killing marine life
38
Q

Environmental concerns: recycling

A
  • reduces environmental impacts by conserving finite fossil fuels
  • Decreasing the amount of waste going to landfill
39
Q

process of recycling polymers

A

The recycling process is undermined if polymers are mixed as this renders the products unstable

  • sorted
  • chopped into flakes
  • washed
  • dried
  • melted
40
Q

Environmental concerns:
PVC recycling
why is disposal hazardous?
What happens when they are burned?

A
  • high chlorine content and range of additives present in the polymer
  • releases hydrogen chloride, corrosive gas and other pollutants like toxic dioxins
41
Q

What does new technology involving PVC’s do?

why is it useful?

A
  • use solvents to dissolve the polymer
  • High grade PVC is then recovered by precipitation from the solvent,

-It is useful because the solvent can be used again

42
Q

Environmental concerns:

Using polymers as fuel

A
  • Waste polymers can be incinerated to produce heat , generating steam to drive a turbine producing electricity
43
Q

Why are some polymers difficult to recycle?

A
  • As they are derived from petroleum or natural gas, they have a high stored energy value
44
Q

Environmental concerns:

Feedstock recycling

A
  • describes the chemical and thermal processes that can reclaim monomers, gases, or oil from waste polyymers
45
Q

what can the products of feedstock recycling be used as?

A
  • As raw materials for the production of new polymers
46
Q

What is a major advantage of feedstock recycling?

A
  • It is able to handle unsorted and unwashed polymers

spirit burner (chemistry) (4 decks)

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