DF 6 - alkenes, versatile compounds Flashcards
specification reference - (b) (m) (o) (q)
what is the difference between saturated and unsaturated
Saturated hydrocarbons contain only single covalent bonds while unsaturated hydrocarbons contain at least one or more double or triple carbon-carbon bonds.
which contain more saturated fats
animal fats such as butter and lard
which contain more unsaturated fats
plant derived fats and oils
why are saturated fats associated with heart disease
they were thought to cause a buildup of cholesterol in the arteries
which types of fats cause heart disease
trans fats
what does cis and trans refer to
the arrangement of the groups across the carbon=carbon double bond
what type of fats are naturally occuring unsaturated fats
cis fats
why aren’t cis fats used in the food industry
they often have melting points that are slighltly too low for use in the food industry
how did cis fats turn to trans fats
partial hydrogenation was carried out to react the fats with hydrogen in order to saturate some of the unsaturated bonds. in the process some of the cis compounds were turned to trans
why are trans fats being phased out
they are found to likely lead to a buildup of cholesterol
what is an alkene
unsaturated hydrocarbons with a double C=C bond.
what is the general formula of non cyclic alkenes
CnH2n (same as cycloalkanes but the double bond makes alkenes react very differently from cycloalkanes)
rules for naming alkenes
1) identify the longest carbon chain and use the appropiate prefix to the number of carbons
2) follow prefix ‘ene’ to indicate the compound is an alkene
3) for alkenes, where longest carbon chain is four carbon atomd or longer insert a number before ‘ene’ to indicate the location of the C=C double bond
why are alkenes more reactive than alkanes
because of the double C=C bond
which is stronger - a double bond or single bond
double bond
what is the single bond between two carbon atoms in alkanes called
a sigma bond σ
what is the double bond in an alkene made up of
a sigma bond σ and a pi bond 𝛑
how are σ bonds formed
by the head-on overlap of s orbitals. the first bond between two atoms in a molecule is always a sigma bond
how are the electrons arranged in a single bond
the two electrons are arrnaged between the atoms in an area of increased electron density
how are 𝛑 bonds formed
by the sideways overlap of p-orbitals. these bonds occur in addition to a sigma bond (double, triple bonds)
what is the bonding like in a
- single bond
- double bond
- triple bond
single bond - a sigma bond
double bond - a sigma bond and a pi bond
triple bond - a sigma bond and two pi bonds
which are stronger 𝛑 or σ bonds
σ bonds
what happens to the bonds when an alkene reacts
the 𝛑 bond breaks but the σ remains intact
what is an addition reaction
where two or more molecules react to form a single, larger molecule
what takes part in an addition reaction
alkenes take part in addition reactions. a small molecule is added across the double bond, causing the 𝛑 bond to be broken
what happens in an addition reaction
the 𝛑 bond breaks. two reactant molecules combine to form one product molecule. an unsaturated alkene forms a saturated molecule
does the saturated molecule have to be an alkane
no. its a molecule with no double bonds
give the equation for a basic addition reaction of an alkene
\ / X Y
C=C + X-Y —> | |
/ \ —C —–C—
| |
what is the addition of hydrogen also called
hydrogenation
what happens in hydrogenation
a mixture of hydrogen gas and a gaseous alkene is passed over a nickel catalyst and at a temperature of 150°C and a pressure of 5atm
the hydrogen adds across the double bond and an alkane is formed
example of hydrogenation
ethene + H2 —> ethane
what is the addition of halogens called
halogenation
what happens in halogenation
alkenes react rapidly with halogen such as chlorine and bromine at room temperature. the halogen adds across the double bond to give a halogenoalkane. (if its chlorine, its chlorination - bromine, then bromination)
example of halogenation
ethene + Br2 —>
1,2 - dibromoethane
how do we test if something is unsaturated
when bromine is added to a sample containing an alkene, it changes from orange to colourless. the colour change shows that the bromine has been added across the double bond. the reactant is used as a test for unsaturation and proves the presence of a double bond
what happens in the addition of hydrogen halides
a hydrogen halide such as HCl, HBr and HI add across the double bond to form a halogenoalkane. happens at room temperature
example of addition of hydrogen halides
ethene + HBr —> bromoethane
HBr is in solution in a polar solvent
what is the addition of steam sometimes called
hydration of steam
what happens in hydration of steam
steam and the gaseous alkene are to a high temperature and pressure in the presence of a phosphoric acid catalyst. 300°C and 60atm
when is hydration of steam to alkenes used
the addition of steam to alkenes is one method of preparing alchohols. this reaction is widely used in manufacturing
example of hydration of steam
ethene + steam (H2O) ———–
phosphoric acid catalyst (H3PO4)
———-> ethanol (alcohol)
how else is ethanol made
in the lab ethene can be converted to ethanol by first adding concentrated sulfuric acid and then diluting with water
what does the addition to unsymmetrical alkenes result in
forms a mixture of organic products
example of addition to unsymmetrical alkenes
propene + hydrogenbromide
- forms a major product = 2-bromopropane
- forms a minor product = 1-bromopropane
what do the major and minor products refer to
the relative amounts of the products formed, based on the stability of the carbocation intermediate during the reaction
the major product is formed in greater quantity
the minor product is formed in a smaller quantity
what is Markownikov’s rule
in the addition of a hydrogen halide to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms and the halogen attaches to the carbon with fewer hydrogen atoms.
what does electrophile mean
a positive ion or molecule with a partial positive charge that will be attracted to a negatively charged region and react by accepting a lone pair of electrons to form a covalent bond
what is the charge of an electrophile
positive/neutral
why does the region between two carbon atoms in ethene have a high density of negative charge
there are four electrons in the double bond of ethene giving it a higher negative charge
what is attracted to the negative charge of the c=c double bond
positive ions or molecules with a partial positive charge on one of the atoms will be attracted to this negatively charged region
how do positive ions react with the negatively charged region of the C=C bond
by accepting a pair of electrons from the C=C double bond
what is the test for unsaturation
when ethene is bubbled through bromine, the red brown bromine becomes decolourised
what is a reaction mechanism
logically deciding movement of electrons using ideas of bond polarity and charge
what happens when bromine for e.g. approaches an alkene
bromine molecules become polarised as it approaches the alkene. the electrons in the bromine are repelled by the alkene reactions and are pushed back along the molecule.
the bromine atom nearest the alkene becomes slightly positively chargedd and the bromine atom further away from the alkene becomes slightly negatively charged.
what happens to the slightly positive bromine atom
it behaves like an electrophile and reacts with the alkene double bond
what happens to the other carbon atom after the bromine adds to the other one
it now only has six outer electrons and has become positively charged. it is a carbocation
what is a carbocation
an ion with a positively charged carbon atom. they react very rapdily with anything that has electrons in it
what happens to the carbocation (bromine)
a pair of electrons move from the bromide ion to the positively charged carbon to form a new carbon-bromine covalent bond
what does electronegative mean
a measure of an atom’s ability to attract shared electrons to itself.
which atoms are more electronegative
On the periodic table, electronegativity generally increases as you move from left to right across a period and decreases as you move down a group.
out of these elements which is the least and most electronegative
HF
HCl
HBr
HI
HF slowest reaction/strong
most electronegative
HCl
HBr
HI fastest reaction/weak
least electronegative
what does phosphoric acid look like
H
\
O
|
O - P - O
/ || \
H O H
what makes phosphoric acid polar
it has electronegative elements
which elements are δ− and which are δ+ in phosphoric acid
oxygen is δ-
hydrogen is δ+
describe the conditions needed for hydration of alkenes such as ethene
requires phosphoric acid catalyst at temperature 300°C and 60atm and water in the form of steam
what is stage 1 of the hydration of steam with ethene mechanism
stage 1 - pair of electrons in the 𝛑 bond of ethene are attracted to one of the δ+ hydrogen atoms in phosphoric acid.
- electrons in the pi bond form a covalent bond with the positive hydrogen atom. the covalent bond between hydrogen and oxygen breaks.
- pair of electrons move to oxygen atom
what is heterolytic fission
when a covalent bond breaks, one bonding atom receives both electrons from the bonded pair.
what is a carbocation intermediate
a positively charged intermediate on the carbon atom that has not yet formed a bond.
what forms at the end of stage 1 of the hydration of steam
a carbocation intermediate with a positive carbon atom forms because of stage 1. we also have a dihydrogen phsophate ion with a negatively charged oxygen atom
what is stage two of the hydration of steam with ethene mechanism
water in the form of steam reacts with the carbocation intermediate. the oxygen in a water molecule has two lone pairs of electrons. one lone pairs forms a covalent bond between the oxygen and the positive carbon in the carbocation intermediate. this leaves the oxygen to become positive as one of its lone pairs has formed a covalent bond
what is stage 3 of the hydration of steam with ethene mechanism
the dihydrogen phosphate ion forms a covalent bond to a hydrogen (from the water molecule). at the same time, the covalent bond in the intermediate breaks by heterolytic fission. the pair of electrons move to the oxygen
what is stage 4 of the hydration of steam with ethene mechanism
ethanol is formed and the phosphoric acid catalyst is regenerated.
what experimental evidence supports the mechanism for electrophillic addition via a carbocation
if hex-1-ene reacts with bromin in the presence of chloride ions, two products are formed: 1,2-dibromohexane and 1-bromo-2-chlorohexane
1,2-dichlorohexane is not formed as the electrophile must attack first then the anions (chloride ions)
what does the test for an alkene use instead of pure bromine
bromine water
what does bromine water do to the reaction with an alkene
water molecules have lone pairs of electrons and act as nucleophile in competition with bromine ions. this forms a bromoalcohol
what would happen if the bromine water was dilute
there would be too many water molecules than bromine ions present and the bromoalcohol will be the main product of the reaction. the bromine water is still decolourised
