Alkenes Flashcards
What are alkenes?
(structure, bonding, reactivity)
- Alkenes are unsaturated (can make more bonds with extra atoms in addition reactions) hydrocarbons wih a double covalent (C=C) bond.
- They have a high electron density; with two pairs of electrons in the C=C double bond; they’re pretty reactive, more so than alkanes, susceptible to attack by electrophiles (positively charged species that are attracted to an electron rich centre/region; an electron pair acceptor).
What orientation do alkenes take, and why is this so?
- Alkenes are a planar (flat) molecule due to the >C=C< arrangement.
- There is no rotation around the double bond due to the prescence of the π orbital; restricted rotation (consisting of a p-orbital [single electron] on each carbon that overlaps to form one single orbital with a cloud of electron density above and below the single bond; the π orbital)
What type of isomerism can alkenes exhibit, and why?
- Position isomerism; where the position of the double bond changes; e.g. but-2-ene, but-1-ene.
- Geometrical isomerism; E-Z stereoisomerism (compounds have the same structural formula with atoms/bonds/groups being arranged differently in space‘isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space’)
- Arises from the restricted rotation around the C=C bond.
Which groups go where on an E-isomer, and a Z-isomer? Name it.
- E-isomerism; same groups are across the double bond on opposite sides; E for ‘entgegen’.
- Z-isomerism; same groups are both above or below the double bond; Z for ‘zusammen’.
What type of reactions occur to alkenes?
- Electrophilic addition reactions:
- Electrophile is attracted to the double bond
- Electrophile (with a positve charge; ion or polar molecule) accepts a pair of electrons from the double bond; double bond opens, atom added to the C.
- Positive ion (carbocation) is formed with the main group, the negatively charged ion (once was electrophile) forms a bond with the carbocation.
What is the test for unsaturation? What is the mechanism?
Draw.
- Bromine solution (bromine water usually) is used to test for unsaturation; a test for the C=C double bond.
- It occurs via the mechanism of electrophilic addition; when bromine is added to an alkene, the double bond breaks, and a bromine atom becomes attached to each carbon. The bromine loses its original red-brown colour (decolourised) to give a colourless liquid.
- Double bond repels electrons in the Br-Br bond, polarising Br2.
- Two electros from the C=C bond form a bond with the Brδ+ and the other becomes a Br- ion taking initial Br2 bonding electrons.
- Carbocation formed; carbon NOT bonded to the bromine has the postive charge.
- Br- ion now forms a bond with the carbocation, giving 1,2-dibromoehtane.
What is the mechanism of electrophilic addition of ethene with HBr? (hydrogen halide reaction)
- Double bond breaks and a hydrogen atom ends up attached to one of the carbons and a bromine atom to the other.
- Bromoethane is formed.
- Bromine is more electronegative than hydrogen; Hδ+- Brδ- polar bond.
- Electrophile is the postive Hδ+.
- Attracted to the C=C bond due to high electron density there.
- One of the C=C electron pairs bonds with Hδ+ to form a carbocation, whilst bonding electrons form H-Br go to Brδ-.
- Bond in H-Br breaks heterolytically; both electrons from the shared pair in the bond goes to the Br atom as it is more electronegtaive than H, leaving a Br- ion.
- Br- ion then attaches to the carbocation forming a bond with one of its electron pairs.
What is the mechanism of electrophilic addition of ethene with H2SO4?
Alcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst, such as sulfuric acid.
- Cold concentrated sulfuric acid reacts with an alkene in an electrophilic addition reaction.
- Adding cold water and then warming the product hydrolyses it to form an alcohol.
- Sulfuric acid is reformed; catalyst.
How do you predict the products of addition to asymmetrical
alkenes?
(reference to the relative stabilities of primary,
secondary and tertiary carbocation intermediates)
- Adding hydrogen halides to asymmetrical (not symmetrical; double bond is not in the exact middle of the chain) alkenes gives rise to two possible products: primary carbocation (least stable) and/or secondary carbocation (more stable).
- The amount of each product formed depends on how stable the carbocation fromed in the middle of the reaction is:
- Carbocations with MORE alkyl groups are more stable as the alkyl groups have a positive inductive effect; feeding electrons towards the postive charge.
- The electron releasing effect tends to stabilise the postive charge of the intermediate carbocation.
- The more alkyl groups; the more stable carbocation, the more likely this product forms.
E.g., propene + hydrogen bromide is likely to form much more 2-bromopropane than 1-bromopropane.
What are the typical conditions for the industrial production of ethanol from ethene?
- Ethanol is manufactured by hydratomg ethene with steam. The reaction is reversible, and the formation of the ethanol is exothermic.
- Only 5% of the ethene is converted into ethanol at each pass through the reactor. By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion.
- 300oC
- 60atm
- phosphoric(V) acid catalyst H3PO4
How are addition polymers are formed from alkenes?
- Alkenes form the monomers of the basis of addition polymers; the double bonds are opened up and joined together to form a long chain; a polymer.
- The reaction is addition polymerisation.
How reactive are polymers/poly(alkenes)?
- The polymers, like alkanes, are unreactive, due to the loss of the double bond, effectively being an alkane.
- The chains may be straight or have some degree of branching.
How do you recognise the repeating unit of a polymer/poly(alkene)?
- The repeating unit of a polymer is the smallest group of atoms that produce the polymer when repeated over and over; shown in brackets, with a subscript ‘n’.
- It can be found by substiuting the C=C double bond in a monomer for a single C-H bond.
What are the typical uses of poly(ethene) and poly(propene)?
Different polymer structures have different properties; hence different uses.
Poly(ethene); soft, flexible.
Used for: plastic bags, sqeezy bottles, washing-up bowls.
Poly(propene); tought, strong.
Used for: bottle crates, rope.
Is poly(propene) is recycled?
Problems with plastics.
Yes.
Many polymers are difficult to dispose of (not biodegradable) and are made from non-renewable oil fractions, so it makes sense to recycle and re-use them.
Poly(propene) is a thermoplastic polymer; it will soften when heated so it can be melted and re-used. However, this can only be done a limited number of times; at each heating some of the chains break and become shorter thus degrading the plastic’s properties.
