Alkenes Flashcards by Nelia Clarke

Describe the structure and bonding in alkenes

Structure: trigonal planar , 120°

Bonding : double carbon bond (pi and sigma)

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Explain what a pi bond is

Side to side overlap of p orbitals in plane above and below sigma bond

Electron rich centre

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State the general formula of alkenes

CnH2n

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Why might the formula of an alkene not fit the general formula?

Diene or cyclic

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Define stereoisomerism

Same structural formula but different orientation in space

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Why does stereoisomerism not arise in alkanes?

Don’t have pi bond which restricts rotation

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What must a compound have to display stereoisomerism?

Double carbon bond

Two different groups attached to each carbon atom involved in double bond

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Give the types of E/Z isomerism

E= trans (H on opposite sides of c=c)

Z= cis (H on same side of c=c)

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How do you determine if a compound has E/Z(cis/trans) isomerism?

Has c=c
Has two different groups on each carbon atom

Highest priority group on opposite sides = E

On Same side= Z

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How do you determine if a compound has cis/trans isomerism?

Has one H bonded to each carbon atom

Cis= H on same side
Z=H on opposite sides

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How is priority assigned to groups?

Atom with highest atomic number (proton)

If the same do next atom along with highest atomic number

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What is the name for the method of assigning priority?

Cahn-ingold-prolog principle

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Do E or Z isomers have a higher BP, why?

Fit more closely and compact so have more surface contact points so London forces are stronger

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Describe and explain the reactivity of alkenes

Fairly reactive

In pi bond e are above and below the plane of the sigma bond so more exposed

Pi have a low bond enthalpy so are easily broken

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State the addition reactions possible with alkenes

Hydrogenation

Elecrrophillic addition:
Halogenation
Addition of hydrogen halide

Hydration

Polymerisation

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State conditions, products and reagents for hydrogenation

Nickel catalyst

Hydrogen

Alkane

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State conditions, products and reagents for hydration

Acid catalyst

Steam (water at high temp)

Mixture of alcohols

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Describe the test for alkenes

Add bromine water

Positive= orange to colourless

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What is Halogenation?

Electrophillic addition

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What is an electrophile?

Species that accepts a pair of electrons

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Describe electrophillic addition of Br2

Br2 approaches double bond

Electron rich Centre induces dipole

Pi bond breaks donating 2xe to positively partially charged Br

Br2 breaks via heterolytic fission producing two ions

Br+ bonds to carbon producing intermediate (carbonation)

Br- bonds to carbocation by donating lone pair of electrons to form drive covalent bond

haloakane is formed

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Define carbocation

Positively charged carbon

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What do curly arrows show?

Movement of e from donator to reciever

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At what temperature does electrophillic addition occur at?

Rapid at room temperature

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How is a liquid alkene and gaseous hydrogenhalide reacted?

Gas bubbled through

How is a liquid alkene and liquid hydrogenhalide reacted?

Mixed

How is a gaseous alkene and gaseous hydrogenhalide reacted?

Mixed

Name the two types of product that form from the reaction between an unsymmetrical alkene and hydrogenhalide

Major and minor

How are major and minor products classified?

Major = most stable (highest carbocation)

How are carbocations classified?

The number of carbon atoms bonded to carbocation 1° 2° 3°

What carbocation is the most stable and why? (name effect)

3° because has the most alkyl groups which push electrons towards c+ stabilising it more + is spread over alkyl groups so more alkyl groups = more stable Inductive effect

What is markownikoffs rule?

Hydrogen in hydrogen halide will bond to the carbon with the most hydrogens in an unstable alkene to produce the most stable carbocation and major product

Outline a Polymerisation reaction

Monomers are joined to form a polymer with identical repeating units

State the conditions in polymerisation

High temp and pa Catalyst

What are the environmental concerns of polymers?

Not very reactive so don't degrable easily Persist in environment= destroy marine life

State the different methods of processing polymers

Recycling Chemical feedstock Energy production

Describe how polymers are recycled

Polymers have to be sorted Chopped into flakes Washed, dried and melted to produce pellets Used to produce more polymer products

Describe how polymers are used for energy production

Polymers are burnt and release lots of energy Energy boils water to steam which turns turbines to generate electricity

Why might polymers be use dto produce electricity ?

Can't be recycled

Describe how polymers are Used as chemical feedstock

Polymers don't have to be sorted Chemical and thermal processes are used to produce oil gas or monomers Use to produce new polymers

What is the advantage of using polymers for chemical feedstock and not recycling?

They don't have to be sorted

What's the difference between chemical feedstock and recycling?

In Chemical feedstock the polymers don't have to be washed Raw materials are produces in chemical feed stock but polymer pellets in recycling

What is a bioplastic?

A plastic made from starch, cellulose, plant oil or proteins

What are the advantages of using bioplastic?

Don't produce toxic byproducts when degraded Conserve oil reserves Sustainable and renewable

Define biodegradable

Degraded by micro-organisms to produce co2 water and biological compounds

What are biodegrable plastics usually made of?

Cellulose or starch

What are the advantages of using biodegrable plastics?

Produce no toxic byproducts

What are photodegradable plastics?

Plastics where the bonds are weakened by light to start the degradation process

Why are photodegradable plastics not as environmentally friendly as biodegrable plastics?

May be made of oil (not sustainable, renewable and may produce toxic byproducts)

What is a polymer?

A large molecule made of many identical repeating units called monomers

What is a Co-polymer

A polymers made of at least 2 different monomers

Describe the difference between the strength of long and short polymers

Longer = stronger

Describe and explain how branching affects strength of polymers

Less branching = stronger as more compact

What is cross-linking?

Covalent bonds between chains

Define thermosetting polymers

Polymers with cross links

Describe thermosetting polymers

Retain shape when heated

Define thermoplastic polymers

Have no crosslinks, only intermolecular forces

Describe thermoplastics

Change shape when heated (remoulded)

Describe and explain the effect of crosslinks on polymers

More crosslinks = strogner and so don't change shape as much when heated

Why does cyclo hexane have a BP than cyclopropane

More carbon atoms means more surface contact points means stronger LF which require more energy to break