alkenes chp 13 Flashcards
what is the structure of an alkene
- contain at least 1 carbon-carbon double bond
what is the general fromula for aliphatic alkenes
aliphatic alkenes that contain 1 double bond have the general formula CnH2n
how many electrons does carbon have in its outer shell
each carbon atom has 4 electrons in its outer shell and can use these electrons to from bonds
which orbital is the electron that froms the pi bond in an alkene in
the P-orbital
How is a pi bond formed
a pi-bond is formed by the sideways overlap of 2 p-orbitals, in alkenes 1 from each carbon atom of the double bond
where is the electron density concentrated in a pi-bond
the pi electron density Is concentrated above and below the line joining the nuclei of the bonding atoms
How does a double bond effect a molecules geometery
the pi-bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond
what is the shape around the double bond
the shape around each of the carbon atoms in the double bond is trigonal planar
why is the shape around a double bond the way it is
- 3 regions of electron density repel each other as far apart as possible, so the bond angle around each carbon atom is 120 degrees
what is steroisomerism
same molecular formula but different arrangement of atoms in space
what are the 2 types of stereoisomerism and when do they occur
there is E/Z isomerism and optical isomerism.
-E/Z isomerism only occurs in compounds with a c=c double bond,
-optical isomerism can occur in a much wider range of compounds, including alkanes with no functional groups
why does E/Z isomerism arise
stereoisomerism around double bonds arises because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
what causes the rigidity of double bonds
the reason for the rigidity is the position of the pi-bond’s electron density above and below the plane of the sigma-bond
what conditions have to be met for E/Z isomerism to occur
- C=C double bond
- different groups attached to each carbon atom of the double bond
what is cis-trans isomerism and when is it used
-cis-trans isomerism is the name commonly used to describe a special case of E/Z isomerism.
-in cis-trans isomers one of the attached groups on each carbon atom of the double bond must be hydrogen
match up the cis and trans to the corresponding E and Z
the cis isomer is the Z isomer
the trans isomer is the E isomer
how do you use cahn-ingold-prelog nomenclature
the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number
-if the groups of higher priority are on the same side of the double bond, the compound is the Z isomer
-if the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer
whats the difference between E and Z isomers
The Z-isomer (zame) has the groups with priority together, either above or below the carbon, carbon double bond. The E-isomer occurs when the groups with priority are on opposite sides of the double bond.
how do you assign priority to atoms attached to double bond carbons
-examine the atoms attached directly to the carbon atoms of the double bond, the higher the atomic number, the higher the priority
-if the 2 atoms attached to a carbon atom in the double bond are the same, then you will need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority
are alkenes more or less reactive than alkanes and if so why
alkenes are much more reactive than alkanes because of the presence of the pi bond
what is the most common type of reaction that alkenes undergo
alkenes undergo addition reactions relatively easily
explain in terms of bonding why alkenes are able reactive
being on the outside of the double bond, the pi-bond are more exposed than the electron in the sigma-bond, pi-bonds breaks more readily allowing for reactions to occur
what are the most common addition reactions that alkenes undergo
-hydrogen in the presence of a nickel catalyst
-halogens
-hydrogen halides
-steam in the presence of an acid catalyst
explain the hydrogenation of alkenes
when an alkene is mixed with hydrogen and passed over a nickel catalyst at 423 K, an addition reaction takes place to form an alkane
-all C=C bonds react with hydrogen in this way
features of halogenation of alkenes
- occurs readily
- occurs at room temperature
- occurs with all halogens
how do you test for unsaturated hydrocarbons
-the reaction of alkenes with bromine can be used to identify if there is a C=C bond present
-when bromine water (an orange solution) is added drop wise to a sample of an alkene, bromine adds across the double bond, the orange colour disappears, indicating the presence of a C=C bond. If the same test is carried out with a saturated compound, there is no addition reaction and no colour change
-any compound containing a C=C bond will decolourise bromine water
explain how the alkenes react when in different states of matters to undergo addition reactions of hydrogen halides
-Alkenes react with gaseous hydrogen halides at room temperature to from haloalkanes.
-if the alkene is a gas then the reaction takes place when 2 gases are mixed
-if the alkene is liquid then the hydrogen halide is bubbled through it
-alkenes also react with concentrated concentrated hydrochloric acid (HCl) or hydrobromic acid (HB) which are solutions of the hydrogen halides in water
-you can get different products depending on what alkene is used
Explain the hydration reactions of alkenes
alcohols are formed when alkenes react with steam, in the presence of a phosphoric acid catalyst (H3PO4)
-steam adds across the double bond
-there are multiple products you can recieve from this reaction depending on the alkene
What are some modern applications for hydration reactions of alkenes
Used widely in industry to produce ethanol from ethene
what mechanism is most common for alkenes to undergo
electrophillic addition
what does the high electron density of the pi-electrons attract
electrophilles
what are electrophiles
- species that is attracted to electron-rich areas
- accepts electron pairs
- usually cation or molecule containing positive dipole
draw and electrophilic addition between but-2-ene and hydrogen bromine
Draw the electrophilic addition reaction mechanism between but-2-ene and bromine
what is markownikoff’s rule
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 the smaller number of carbon atoms attached to it
whats the difference between the types of carbocations
primary carbocations are only attached to 1 other carbon
secondary are attached to 2 other carbons
tertiary are attached to 3 carbon atoms
how are carbocations classified
classified by the number of alkyl groups attached to the positively charged carbon atom
how are alkyl groups usally represented
-R
what is the order of stability of carbocation from most stable to least
-tertiary carbocations ( 3 -R groups) are the most stable,
-secondary carbocations ( 2 -R groups) are the next most stable,
-Primary ( 1 -R group) is the lest stable
what is carbocation stability linked to and explain it
- it is linked to the electron-donating ability of alkyl groups.
- Each alkyl group donates and pushes 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
what are polymers
extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers
-addition polymers have high molecular masses
what type of polymerisation do alkenes undergo
alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds
what conditions are used for industrial polymerisation
high temperatures and high pressures also using catalysts
how would you figure out what monomer is used for a certain polymer
synthetic polymers are usually named after the monomer that reacts to from their giant molecules, prefixed by “poly”.
how would you draw a monomer uint and a repeat unit of a polymer
-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
what is a repeat unit
a repeat unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again
what are some common polyerms and what are the uses
explain how to recycle polymers
once sorted, the polymers are chopped into flakes, washed, dried and melted. the recycled polymer is cut into pellets and used by manufacturers to make new products
what are the benefits of recycling polymers
reduces their environmental impact by conserving finite fossil fuels as well as decreasing the amount of waste going to landfill
why is recycling PVC dangerous
- hazardous due to high chlorine content and range of additives present in polymer.
- Dumping PVC in landfill is not sustainable
- when burnt PVC releases hydrogen chloride a corrosive gas
how is PVC recycled
-solvents are used to dissolve the polymer.
-High-grade PVC is then recovered by precipitation from the solvent, and the solvent is used again
how are waste polymers used as fuel
some waste polymers are derived from petroleum or natural gas, they have a high stored energy value. waste polymers can be incinerated to produce heat, generating steam to drive a turbine producing electricity
what is feedstock recycling
Feedstock recycling describes the chemical and thermal processes that can reclaim monomer, gases or oil from waste polymers. The products from feedstock recycling resemble those produced from crude oil refineries
what can the products of feedstock recycling be used for
can be used as raw materials for the production of new polymers
what is a major advantage of feedstock recycling
A major advantage of feedstock recycling is that it is able to handle unsorted and unwashed polymers
what are biodegradable made from
usually made from starch or cellulose, or contain additives that alter the structure of traditional polymers so that micro-organisms can break them down.
what are biodegradable polymers broken down into
broken down by microorganisms into water, carbon dioxide and biological compound
what are compostable polymers made of and what are they broken down in to
compostable polymers are based on poly(latic acid), compostable polymers degrade and leave no visible or toxic residues
what are some uses for compostable polymers
supermarket bags made from plant starch can be used as bin liners for food waste so that the waste and bag can be composed together.
what are photodegradable polymers
photodegradable oil-based polymers contain bonds that are weakened by absorbing light to start the degradation, light-absorbing additives are used
