Alkenes Flashcards
What type of bonds do alkenes have
double carbon bond
What is the formula
CnH2n
Are they saturated or unsaturated
unsaturated hydrocarbons
what is the functional group
double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot
when should you state the position of the double bond
In molecules with a straight chain of 4 or more carbon atoms
how is a π bond formed
the sideways overlap of adjacent p orbitals
The two lobes that make up the π bond lie above and below the plane of the σ bond. Resulting in no free rotation of the groups about the C atoms
single π bond is drawn as two electron clouds, which represent one bond containing two electrons
What are sigma bonds
end to end overlap of atomic orbitals
The electron density in a σ bond is symmetrical between the nuclei of the atoms forming the bond
The pair of electrons is found between the nuclei of the two atoms
explain the sigma and pi bonds in Ethene
Each carbon atom uses three of its four electrons to form σ bonds
Two σ bonds are formed with the hydrogen atoms
One σ bond is formed with the other carbon atom
The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
This means that the C-C is a double bond: one σ and one π bond
what area is of high electron density in alkenes
double bond as four electrons found here making it susceptible to attack by electrophiles
what is electrophilic addition
addition of an electrophile to a double bond. The
C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms
what are examples of electrophilic addition
Hydrogen (also known as hydrogenation reaction)
Steam (H2O (g))
Hydrogen halide (HX)
Halogen
What is the hydrogenation reaction
alkene + H2 –> Alkane
(Nickel catalyst)
what is the reaction with steam
alkene + H2O –> alcohol
(H3PO4)
reaction with hydrogen halide
alkene + HBr –> Halogenoalkane
reaction with halogen
alkene + halogen –> halogenoalkane
how is margarine formed
Naturally occurring vegetable oils are unsaturated and contain C=C double bonds
When these bonds react with hydrogen, some of the C=C double bonds become C-C single bonds
This process changes the properties of vegetable oil and converts it into a solid: margarine
How can alkenes be oxidised
by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent
Explain the process of oxidation of alkenes
When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol. distinguish alkanes from alkenes ( alkanes do not have double bonds and so are not oxidised in this way)
equation for oxidation of alkene and where two OH are come from
The potassium manganate provides an oxygen atom (oxidation)
Then water in the solution provides another oxygen atom and two hydrogen atoms, so there is addition of two OH groups across the double bond
what is heterolytic fission
breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaving behind a positive ion
Are hydrogen halides polar
are polar as the hydrogen and halogen atoms have different electronegativities
The bromine atom has a stronger pull on the electrons in the H-Br bond
As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
What becomes the electrophile from HBr
Hydrogen as it is electron deficienct and accepts a pair of electrons from the C-C bond in the alkene
How does the H-Br bond break
breaks heterolytically, forming a Br- ion
This result in the formation of a highly reactive carbocation which reacts with the Br- (nucleophile)
How can you identify which product is major or minor
look at the carbon cation whichever one is bonded to most carbons is more stable so more likely to form hence the major product from the nucleophilic attack
are halogens polar
are a non-polar molecules as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond
what happens when bromine gets close to alkene double bond
the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom
As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged creates induced dipole
What is the saturation test
Halogens can be used to test if a molecule is unsaturated.
Br2(aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used
The unknown compound is shaken with the bromine water
If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise
carbon cation
a positively charged carbon atom
What is addition polymerisation
the reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product
how does addition polymerisation work
Just like in other addition reactions of alkenes, the π-bond in each C-C bond breaks and then the monomers link together to form new C-C single bonds
what is a polymer
long-chain molecule that is made up of many repeating units
what is a monomers
The small, reactive molecules that join together to form the polymer
why are polymers unreactive
The addition polymers made from alkenes are saturated compounds because they do not contain double bonds and the main carbon chain is non-polar
what is a repeating unit
portion of the whole molecule that repeats itself several times. A polymer consists of lots of repeating units joined together. The repeating unit has a similar structure to the monomer, except it has the double bond opened up.
describe the reactivity and non-biodegrade polymers
The low reactivity of many polymers makes them ideal for certain uses, e.g. food packaging, but at the same time creates problems with their disposal as a lot of polymers are non-biodegradable
explain how recycling of polymers work
Polymer recycling reduces the amount of waste that it going to landfill sites
Newer landfill sites can have a recycling point where the new waste is brought before going to into the actual landfill - this is in an effort to reduce the amount of polymers (and other recyclable materials) unnecessarily going into the waste site
what does recycling polymers do
reduce the use of finite resources
Lots of polymers are made from the products of cracking crude oil and it’s fractions
what is a disadvantage of recycling polymers
Recycling polymers is a time-consuming process as they have to be sorted into the different categories
why can polymers be incinerated
Since they have a large amount of energy stored within the polymer chains.
what can incineration do
This process can then be used to boil water and use the water vapour to turn turbines inside a power station, in a similar fashion to coal-fired power stations
does incineration cause environmental pollution
as the carbon within the polymer can be released as carbon dioxide contributing to global warming
Other toxic waste products include hydrogen chloride and other chlorinated molecules from the combustion of PVC
What is feedstock recycling
where waste polymers are broken down, by chemical and thermal processes, into monomers, gases and oils
What are products used for
the raw materials in the production of new polymers and other organic chemicals
benefit of feedstock recycling
compared to other methods of polymer disposal, is that it works with unsorted and unwashed polymers
what are some aims when designing a sustainable polymer manufacturing process
Use chemicals that are as safe and environmentally friendly as possible
Use renewable feedstock chemicals where possible
Reduce the energy requirements for the reaction as well as increase the energy efficiency, which has both an environmental and financial bonus
Improve atom economy
how can you remove toxic waste products
The waste gase from the incinerator are scrubbed/reacted with a base or carbonate
The base reacts with the acidic HCl gas, neutralising it
CaO (s) + 2HCl (aq)→CaCl2 (aq) + H2O (l)
how can biodegradable polymers be broken down
broken down over time by microorganisms
Common products from this process include carbon dioxide, water and other organic compounds
uses of waste plastic
incineration to release energy
recycling
feedstock recycling
what is a stereoisomer
compounds which have the same structural formula but their atoms are arranged differently in space.
what is structural isomer
molecules that have the same molecular formula, but have a different arrangement of atoms
why is incineration not a suitable method of disposal
because it produces HCl which is corrosive and causes acid rain
what is a dipole
when electrons are shared unequally between atoms in the same molecule due to a high difference in the electronegativity of the atoms involved.
why cant LiAlH4 be used to reduce alkenes
because the hydride ion will be repelled by regions of high electron density
how do you spot which carbon is carbocation
the tertiary one as it is more stable and more positively inductive
