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
