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CHEMISTRY Module 4: Core Organic Chemistry > 4.1.3 Alkenes > Flashcards

4.1.3 Alkenes Flashcards

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1
Q

General formula of (aliphatic) alkenes.

A

CnH2n

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2
Q

What are alkenes?

A

Unsaturated hydrocarbons.

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3
Q

Describe the bonding in alkenes.

A

Double bond comprised of one sigma bond (from overlap of orbitals between bonding atoms) and one pi bond (sideways overlap of 2 p-orbitals with one electron from each carbon).
Pi bond causes a lack of rotation around the double bond.

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4
Q

Describe the shape and bond angles in alkenes.

A

Trigonal planar shape due to: 3 regions of electron density around each of the carbons, repel eachother.
Bond angle around each carbon atom is 120.

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5
Q

Definition of stereoisomer.

A

Compounds with same structural formula but different arrangement of the atoms in space.

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6
Q

Definition of E/Z isomerism.

A

Type of stereoisomerism that occurs when there’s a C=C double bond and different groups attached to each carbon of the double bond in a molecule.

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7
Q

Definition of cis-trans isomerism.

A

Special case of E/Z isomerism in which two of the groups attached to each carbon of the double bond are hydrogen atoms.

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8
Q

How are compounds classed as E/Z?

A

E= entgegen, the two higher priority (higher atomic number) atoms are on opposite sides
Z= zusammen, the two higher priority (higher atomic number) atoms are on the same side

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9
Q

How are compounds classed as cis/trans?

A

Cis= the two hydrogen atoms are on the same side
Trans= the two hydrogen atoms are on opposite sides

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10
Q

Why are alkenes much more reactive than alkanes?

A

Presence of a pi bond, which has a lower bond enthalpy than a sigma bond meaning its weaker and therefore broken more easily also the electrons are more exposed because they’re on the outside of the double bond so alkenes can react easily.

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11
Q

Conditions for addition reaction of alkenes with hydrogen.

A

Nickel catalyst and 423K

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12
Q

Products of hydrogenation of alkenes.

A

Alkane

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13
Q

Conditions for halogenation of alkenes.

A

Chlorine/bromine and room temperature.

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14
Q

Products of halogenation of alkenes

A

Dihaloalkane

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15
Q

How can an organic compound be tested for unsaturation?

A

Adding orange bromine water to alkenes makes a colourless solution but there will be no colour change with an alkane.

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16
Q

Conditions for addition reaction of alkenes with hydrogen halides.

A

Hydrogen halide and room temperature.

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17
Q

Products of addition reaction of alkenes with hydrogen halides.

A

Haloalkanes

18
Q

Conditions for hydration reactions of alkenes.

A

Steam and phosphoric acid catalyst (H3PO4)

19
Q

Products of hydration reactions of alkenes.

A

Alcohol

20
Q

Definition of an electrophile

A

Electron pair acceptor

21
Q

Mechanism for when alkenes take part in addition reactions to form saturated compounds.

A

Electrophilic addition.

22
Q

Why are electrophiles attracted to alkenes.

A

Double bond has a region of high electron density due to pi bond which attracts electrophiles.

23
Q

Describe the mechanism for but-2-ene and hydrogen bromide.

A

Bromine more electronegative so HBr is polar and has dipole, bromine - and hydrogen +
Electron pair in pi bond attracted to partially positive H atom, double bond breaks
Bond forms between H atom and carbon atom from double bond
HBr breaks by heterolytic fission, electron pair to bromine
Bromide ion and carbocation formed (+carbon atom)
Br- ion reacts with carbocation to form product.
(Electrophilic addition)

24
Q

Describe the mechanism for propene and bromine

A

Bromine is non-polar, when it approaches an alkene, the pi bond electrons interact with electron in Br-Br
Polarisation of Br2, one end partially+ and the other partially- (induced dipole)
Electron pair in pi bond attracted to +, double bond breaks
Bond formed between bromine and carbon
Br2 bond breaks by heterolytic fission, electron pair to Br-
Bromide ion and carbocation formed
Br- ion reacts with carbocation to form product

25
Q

What is a primary carbocation?

A

Positive charge is on a carbon at the end of a chain.

26
Q

What is a secondary carbocation?

A

Positive charge on a carbon with two carbons attached.

27
Q

How are carbocations more stable?

A

More alkyl groups attached to + carbon, the ion is more stable due to charge being spread out.
Order of stability= tertiary (most), secondary, primary (least)

28
Q

Markownikoff’s rule

A

When unsymmetrical alkene reacts with an unsymmetrical molecule the hydrogen attaches itself to carbon with more hydrogens and smaller number of carbons.
MAJOR PRODUCT

29
Q

What do alkenes undergo to become long saturated chains?

A

Addition polymerisation.

30
Q

What is a repeat unit?

A

Specific arrangement of atoms in the polymer molecule that repeats over and over again.

31
Q

Why are polymers used?

A

They’re readily available, cheap to purchase and unreactive.

32
Q

What are the issues with disposal of polymers?

A

Many are non-biodegradable and their unreactive nature makes them difficult to dispose of.

33
Q

Why is PVC disposal difficult?

A

High chlorine content, range of additives present.
Landfill is not sustainable.
When burnt, hydrogen chloride is released and other pollutants.

34
Q

How is PVC recycled?

A

Solvents dissolve the polymer.
High-grade PVC recovered by precipitation from the solvent, solvent then reused.

35
Q

Why are waste polymers used for fuel?

A

They are derived from petroleum or natural gas and have high stored energy value.

36
Q

How are waste polymers used for fuel?

A

They are incinerated to produce heat which generates steam that drives a turbine producing electricity.

37
Q

What is feedstock recycling?

A

Chemical or thermal processes that can reclaim monomers, gases or oil from waste polymers.

38
Q

How can products of feedstock recycling be used?

A

Raw materials for the production of new polymers.

39
Q

Advantage of feedstock recycling.

A

Able to handle unwashed and unsorted polymers.

40
Q

What are bioplastics?

A

Produced from plant starch, cellulose, plant oils and proteins.
Renewable and sustainable alternative to oil-based products.

41
Q

What are biodegradable polymers?

A

Polymers made form starch/cellulose.
Broken down my microorganisms into water, CO2 and biological compounds.
Leave no visible or toxic residues.

42
Q

What are photodegradable polymers?

A

Polymers that contain bonds that are weakened by absorbing light to start degradation.

CHEMISTRY Module 4: Core Organic Chemistry (7 decks)

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