14.1- ALKENES Flashcards
What type of hydrocarbons are alkenes?
unsaturated
What are alkenes made up of?
carbon and hydrogen only
What do alkenes have that make them alkenes?
one or more carbon-carbon double bonds
What does the double bond in alkenes mean to the number of hydrogens?
alkenes have fewer than the maximum possible number of hydrogen atoms
What property does the double bond give alkenes?
makes them more reactive than alkanes
Why are alkenes more reactive than alkanes?
because of the high concentration of electrons (high electron density) between the two carbon atoms
What is ethene used for?
as the starting material for a large range of products
What products can be formed from ethene? (4)
polymers:
polythene, PVC, polystyrene and terylene fabric
products:
antifreeze and paints
When is alkenes produced in large quantities?
when crude oil is thermally cracked
What is the general formula for alkenes?
CnH2n
What is the shape of ethene?
planar (flat) molecule
What happens to the angles as ethene is a planar molecule?
makes the angles between each bond roughly 120 degrees
Unlike the C-C bond in alkanes, what is there none of in alkenes?
no rotation about the double bond
Why is there no rotation about the double bond?
due to the make up of the double bond
What happens to the shape if any molecules in which a hydrogen atom in ethene is replaced by another atom or group?
it will have the same flat shape around the carbon-carbon double bond
as well as a normal C-C single what else is there?
there is a p-orbital on each carbon
what does each p-orbital contain?
contains a single electron
What do the two orbitals on the carbons do?
overlap to form an orbital with a cloud of electron density above and below the single bond
What is the name of the orbital formed from the two p-orbitals overlap?
called a π-orbital
What does the presence of of the π-orbital mean?
means the bond cannot rotate
What is it sometimes called when the bond cannot rotate?
restricted rotation
What can alkenes with more than 3 carbons form?
isomers
According to what are the isomers names?
IUPAC system using suffix -ene to indicate double bond
What types of isomers can alkenes form? (3)
chain isomers
position isomers
geometrical isomers
What is a position isomer for alkenes?
isomers with the double bond in different positions, that is, between a pair of adjacent carbon atoms in different positions in carbon chain
The longer the chain, what does this mean for the number of isomers?
more possibilities there will be so greater number of isomers
What are geometrical isomers a form of?
is a form of stereoisomerism
What do the two stereoisomers have the same of?
same strucural formula
What is different between the two stereoisomers?
bonds arranged differently in space
Where does the geometrical isomer occur?
only around C=C double bond
What is the isomer with the “priority” on the same side called?
Z- isomer
What is the isomer with the “priority” on different sides called?
E- isomer
How many known organic compounds are there?
a huge amount and increasing all the time
Why did the IUPAC produce rules for nomenclature?
so chemists agree on how isomers named
What was E-Z isomers first known as?
cis-trans isomerism
What prefixes were used instead of Z- and E- ?
cis- and trans- respectively
What was a disadvantage of the older notation?
did not work when there were more than two different substituents around a double bond
what is the E-Z notation based on?
atomic numbers
How are E-Z notations based on atomic number?
look at atoms attached to each of the carbon atoms in double bond
when two atoms of higher atomic number on same side of C=C, isomer described as Z
To what extent does the double bond affect the properties of alkenes?
not greatly i.e boiling + melting points
What are the only intermolecular forces that act between the alkene molecules?
van der Waals forces
As van der Waals forces are the only intermolecular forces that act between alkene molecules, what does this mean?
physical properties of alkenes very similar to those of the alkanes
As the number of carbon atoms increases what happens to boiling and melting point?
melting and boiling point increases
Are alkene soluble in water?
no
How does the double bond make a big difference to the reactivity of alkenes compared with alkanes? (hint- enthalpy)
bond enthalpy is much bigger
Are alkenes or alkanes more reactive?
alkenes
what does the C=C bond form in the molecule?
an electron-rich area in the molecule
What can the electron-rich area in the molecule be easily attacked by?
positively charged reagents
What are the positively charged reagents called?
electrophiles (electron liking)
what are electrophiles?
electron pair
What is an example of a good electrophile?
H+ ion
As alkenes are unsaturated what reactions can they undergo?
addition reactions
What are most of the reactions of alkenes?
electrophilic additions
