Chemistry: alkenes
This flashcard set introduces the key structural features of alkenes, including their general formula and functional group. It explains the concept of unsaturation and highlights the trigonal planar geometry around each carbon in the double bond due to high electron density.
general formula of an alkene
CnH2n
Key Terms
general formula of an alkene
CnH2n
functional group alkene
c=c
what is the double covalent bond a centre of
high electron density
unsaturated
contain one or more carbon-carbon double bonds
what is the shape around each carbon in the c=c double bond
triganol planar
what is the bond angle around each carbon in the c=c double bond
120
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| Term | Definition |
|---|---|
general formula of an alkene | CnH2n |
functional group alkene | c=c |
what is the double covalent bond a centre of | high electron density |
unsaturated | contain one or more carbon-carbon double bonds |
what is the shape around each carbon in the c=c double bond | triganol planar |
what is the bond angle around each carbon in the c=c double bond | 120 |
what forms when the p orbitals overlap in the c=c group | a pi bond |
what does the presence of a pi bond create | restricted rotation about the planar c=c bond |
how are alkenes named | like alkanes based on longest unbranched carbon chain with suffix ‘ene’ |
what must be indicated in alkenes with 4 or more carbon atoms in the longest chain | the position of the c=c bond |
diene | 2 c=c bonds |
triene | three c=c bonds |
what is the difference between open chain alkenes and cycloalkenes | cycloalkenes have 2 less hydrogen atoms than open chain alkenes |
how are cycloalkenes named | according to the largest ring containing the c=c bond |
structural isomers | compound with same molecular formula but different structural formula |
stereoisomers | compounds with the same structural formula but different arrangement of atoms in space |
what type of stereoisomerism is exhibited by alkenes | E/Z isomerism |
z | together |
e | opposite |
why does E/Z isomerism occur | as a result of restricted rotation about the c=c double bond |
what must both carbon atoms of the c=c double bond be for an alkene to exhibit E/Z isomerism | both carbon atoms must be bonded to different groups |
what are cycloalkanes | functional group isomers of alkenes |
how to determine whether an alkene is the E or Z isomer | identify the highest priority group on each C atom of the c=c double bond if both highest priority groups are on same side- z isomer if both highest priority groups are on opposite sides of C=c bond- E isomer |
which atom has the highest priority | the atom with the highest atomic NUMBER |
what happens if two atoms have the same atomic number | next atom n chain should be compared (first point of difference used to assign priority) |
why are alkenes reactive | high electron density of c=c double bond |
what type of reactions do alkenes undergo | addition reactions in which a small molecule is added across C=C group |
what does the addition reaction of an alkene form | a saturated molecule (pi bond breaks) |
electrophile | electron pair acceptor |
why do alkenes react with electrophiles | electrophile are attracted to high electron density of C=C bond |
what occurs during the reaction between alkenes and electrophiiles | covalent bonds broken and formed |
what is produced as a result of the reaction between an electrophile and an alkene | a reactive positively charged intermediate called a carbocation |
what does a mechanism show | the breaking and forming of covalent bonds using curly arrows |
curly arrow | shows movement of pair of electrons |
where must curly arrows start from | a lone pair or a covalent bond |
what is the reagent in the reaction of alkanes with halogens | the halogen eg bromine |
what does the reaction between ethane and bromine produce | 1,2- dibromo ethane |
what observations could be made during the reaction between ethene and bromine | bromine decolourises |
what is the name of the mechanism in the reacton between ethene and bromine | electrophilic addition |
what acts as the electrophile in the reaction between bromine and ethene | bromine |
what does the high electron density in the c=c group do | polarises the bond in the bromine molecule |
why does the br-br bond break | a pair of electrons is accepted by the delta plus br atom |
how does the br-br bond break | heterolytic fission |
what does the lone pair of electrons on the bromide ion form | a new c-br bond with the carbocation |
what is bromine used for | to test for the presence of a C=C group |
what is the reagent in the reaction of alkenes with hydrogen halides (same applies as halogen) | the hydrogen halide |
What do hydrogen halides contain | A permanent dipole, h atom always delta plus |
What is the reagent in the electrophilic addition reaction between sulphuric acid and lakebed | Concentrated sulphuric acid |
Organic hydrogen sulfates are readily hydrolysed with warm water to form… | An alcohol |
Hydrolysis | Breaking a bond using water |
What are carbocations classified as | Primary secondary or tertiary |
What is the classification of carbocations based on | The number of carbon atoms directly attached to the carbon with the positive charge |
What does a primary carbocation have | 1 variable group |
What does a secondary carbocation have | 2 variable groups |
What does a tertiary carbocation have | 3 variable groups |
What do alkyl groups have a tendency to do | Release electrons |
What does the tendency of alkyl groups to release electrons help with | Stabilising the positive charge of carbocations- called the positive inductive effect |
What carbocations are the most stable and why | Tertiary as they have the most alkyl groups attached |
Order of stability of carbocations | Tertiary>secondary>primary Most Least |
What does addition to an unsymmetrical Alkene produce | Two different organic products |
Major product | Produced in the largest amount |
Minor product | Produced in the smallest amount |
What is the yield of a particular product related to | The stability of the carbocation it is formed from |
What is produced when a hydrogen is added across a c=c bond | An alkane is produced (hydrogenation) |
What is hydrogenation used for | Important industrial process e.g. Production of margarine |
What catalyst is used in hydrogenation of alkenes | Nickel |
When are polymers formed | When monomers join together to form a very large molecule |
What do alkenesact as in addition reactions | Monomers, the c=c bond breaks allowing the monomers to add together |
Repeating unit | A specific arrangement of atoms that occurs in the structure over and over again |
What are simple polyalkenes | Unreactive |
What does the backbone of single c-c bonds make polyalkenes | Saturated |
What do the properties of polyalkenes depend on | Their structure and the intermolecular forces between neighbouring polymer chains |
What does polyethene have between neighbouring chains | VDW forces |
How can the properties of polymers like polyethene be modified | By increasing branching which decreases points of contact so the VDW forces are weakened- increased flexibility and lowers boiling point |
What is polychloroethene | Rigid and brittle |
What does polychloroethene contain | Polar c-Cl bonds and therefore has permanent dipole-dipole forces between polymer chains |
Addition of what molecules make polymers like polychloroethene more flexible | Plasticisers |
How do plasticisers make polychloroethene more flexible | They force neighbouring chains apart- allowing them to slide over each other |
Where do plasticisers go when added to polyalkenes like polychloroethene | Get in between neighbouring chains and reduce intermolecular forces |