14. Hydrocarbons Flashcards
Describe the production of alkanes from addition reactions
When hydrogen gas and an alkene are heated and passed over a finely divided Platinum or nickel catalyst.
This process is called hydrogenation and it is exothermic
Describe the production of alkanes from cracking
Large less useful hydrocarbon molecules are broken down into smaller more useful molecules.
The hydrocarbon is heated at high temperatures and passed over an aluminum oxide catalyst
Process is an endothermic reaction
Suggest how cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil
The low-molecular mass alkanes formed make good field and are high in demand
The low-molecular mass alkenes are more reactive than alkanes due to their double bond; this makes them useful for the chemical industry as the starting compounds for making new products
Describe the elimination process from which alkenes can be made
A halogenoalkane is heated with ethanolic sodium hydroxide
The eliminated H+ in HBr reacts with the ethanolic OH- to form water
The eliminated Br- in HBr reacts with Na+ to form NaBr
Describe the dehydration process which produces alkenes
Alcohol vapor is passed over a hot catalyst of aluminum oxide powder concentrated acid as a catalyst.
Describe the process of catalytic cracking to produce alkenes
The cracking of long hydrocarbon molecules in crude oil can also form alkenes.
An aluminum oxide catalyst and high temperatures are used to speed up this reaction, the reaction produces a smaller alkane and alkene molecules
What is an electrophilic addition reaction?
The addition of an electrophile to a double bond and the double bond is broken and a new single bond is formed from each of the two carbon atoms.
What does markovnikov’s rule state
In an addition reaction of a halogen halide to an alkene, the halogen ends up bonded to the most substituted carbon atom.
The hydrogen halide is more likely to bond to the carbon atom which is attached to the greater number of hydrogen atoms
Why are alkanes so unreactive?
Both carbon and hydrogen have the same electronegativities which means they share the electrons in the covalent bond equally. So alkanes are non-polar molecules and have no partial positive or negative charges
Environmental consequences of oxides of nitrogen?
Dissolve in water and react to form acid rain.
Environmental consequences of carbon monoxide?
It is a toxic gas
Describe the 4 electrophilic addition reactions.
- Alkene and hydrogen to form an alkane-
Platinum/nickel catalyst + heat - Alkene and steam to form an alcohol
Phosphoric acid + heat catalyst - Alkene and hydrogen halide to form halogenoalkane
Room temperature - Alkene and halogens to form halogenoalkane
Describe the oxidation reactions of alkenes.
- Alkene + water + cold dilute KMnO4 to form a diol.
- Alkene + hot concentrated KMnO4 to form:
Carbon dioxide
Aldehyde
Carboxylic Acid
Ketone
What are carbocations?
They are positively charged carbon atoms with only three covalent bonds instead of four. There are three types of carbocations; primary, secondary and tertiary.
Why are alkanes unreactive?
They are non polar molecules
They have a strong bond between them which takes a lot of energy to break
