Unit 4: Lesson 10: Alkanes Flashcards
What are the alkanes?
A homologous series. They’re hydrocarbons (only hydrogen and carbon) joined with single covalent bonds.
What do alkanes share?
A general formula, similar chemical properties (due to their functional group) and a trend in physical properties.
What are alkanes used for?
Alkanes are commonly used as fuel. For example, methane cooks your dinner in a gas oven, propane is in the canister that runs your BBQ and a mixture of alkanes make up LPG which heats your radiators.
What is a saturated hydrocarbon? What’s the difference between saturated hydrocarbons and unsaturated hydrocarbons?
Alkanes are saturated hydrocarbons. Saturated hydrocarbons have no double bonds, whereas unsaturated hydrocarbons have double bonds.
Are alkanes saturated or unsaturated?
Saturated.
How does the number of carbons affect the boiling point of alkanes?
Their boiling point increases with the number of carbons.
If an alkane is larger, the stronger the intermolecular forces are and the more energy is required to break them. True or false?
True.
What is the general formula for alkanes?
CnH2n+2
Alkanes with the following number of carbons are called what?
a) 1
b) 2
c) 3
d) 4
e) 5
f) 6
a) Methane
b) Ethane
c) Propane
d) Butane
e) Pentane
f) Hexane
Fill in the general formula for alkanes for these different types of alkanes:
a) Methane
b) Ethane
c) Propane
d) Butane
e) Pentane
f) Hexane
a) C1H2x1+2
b) C2H2x2+2
c) C3H2x3+2
d) C4H2x4+2
e) C5H2x5+2
f) C6H2x6+2
Give the molecular formula for the following types of alkanes:
a) Methane
b) Ethane
c) Propane
d) Butane
e) Pentane
f) Hexane
a) CH4
b) C2H6
c) C3H8
d) C4H10
e) C5H12
f) C6H14
As we move through the alkanes (methane to hexane), what is added to the chain each time?
A -CH2-
What is an alkane isomer?
An isomer has the same molecular formula but a different structural formula. EG, there are isomers of pentane. The compound will have the same number and type of each atom (C5H12 - pentane) but they are arranged differently. Pentane is unbranched, but the isomers can be branched.
Describe the reactiveness of alkanes.
Alkanes lack strong double bonds, meaning that they are fairly unreactive/inert compared to other organic compounds.
What two reactions are alkanes involved in?
Combustion and substitution.
Describe the combustion of alkanes.
Alkanes tend to completely combust, resulting in carbon dioxide, water and energy. It only occurs if there’s plenty of oxygen.
CH4 + 2O2 –> CO2 + 2H2O + energy
2C2H6 + 7O2 –> 4CO2 + 6H2O + energy
C3H8 + 5O2 –> 3CO2 + 4H2O + energy
2C4H10 + 13O2 –> 8CO2 + 10H2O + energy
C5H12 + 8O2 –> 5CO2 + 6H2O + energy
Describe substitution reactions involving alkanes (and halogens).
Halogens and alkanes are quite inert. If UV (ultraviolet) radiation is applied, they react to make a haloalkane and a hydrogen halide. This is called a substitution reaction.
Give an example of a substitution reaction and explain it.
Methane (alkane) reacting with bromine gas (halogen) to make a haloalkane (CH3Br) and a hydrogen halide (HBr).
CH4 + Br2 –> CH3Br + HBr
UV radiation is NEEDED. It gives the energy needed to break the bond between the 2 bromines, leaving 2 very reactive bromines free. One hydrogen on the alkane is substituted (replaced) by one of the bromines. This makes the haloalkane, The remaining hydrogen and bromine bond to make the hydrogen halide. If bromine (orange) and methane are left in sunlight, the gas mixture turns from orange to colourless as bromomethane and hydrogen bromide are made. Ideally, a UV lamp is needed. Sunlight rarely has enough UV for the reaction.
https://i.pinimg.com/736x/2d/cf/bc/2dcfbce711e82d473d8fd06d76846cca.jpg
What will the reactants always be in a substitution reaction?
The reactants will be an alkane + a diatomic halogen molecule (two halogen atoms bonded).
How do you write the haloalkane’s chemical formula in a substitution reaction?
The haloalkane will have the same molecular formula as the alkane with one hydrogen removed and one halogen added.
How do you write the hydrogen halide’s chemical formula in a substitution reaction?
The hydrogen halide will be hydrogen atom followed by one halogen atom. When naming a hydrogen halide, don’t forget to replace the n in the name of the halogen with a d. For example, bromine becomes bromide.
When writing a hydrogen halide’s name, what do we always change the ‘n’ in the name of the halogen to? Give examples.
A ‘d’.
Hydrogen chloriDe. (CH4 + Cl2 –> CH3Cl + HCl)
Hydrogen bromiDe. (C2H5 + Br2 –> C2H4Br + HBr)
Hydrogen fluoriDe. (CH4 + F2 –> CH3F + HF)
Hydrogen chloriDe. (C5H12 + Cl2 –> C5H11Cl + HCl)
What does a haloalkane’s name always start with? Give the 4 ways to start the name of a haloalkane and give 4 examples.
The halogens nickname.
Bromine –> Bromo
Chlorine –> Chloro
Fluorine –> Fluoro
Iodine –> Iodo
Chloromethane
Bromoethane
Fluoromethane
1-Iodopentane
What does the haloalkane’s name always end with? Give 4 examples.
The halogen’s name always STARTS with the halogen’s nickname and ENDS with the alkane.
ChloroMETHANE.
BromoETHANE.
FluoroMETHANE.
ChloroPENTANE.
Remember, when writing haloalkanes, to add the carbon number the halogen is linked to if the alkane has more than 1 carbon. Give some examples.
2-chloropropane (“prop” meaning 3 carbon, the chlorine in this case is linked to the 2nd carbon)
1-fluoropropane (“prop” meaning 3 carbon, the fluorine in this case is linked to the 1st carbon)
1-iodoethane (“ethane” meaning it has 2 carbon, the iodine in this case is linked to the 1st carbon)
fluoromethane (“methane” meaning it has 1 carbon, which means there does not need a number, as there is only one carbon it could be linked to)
What’s a mono-substitution reaction?
Only one hydrogen in the alkane is replaced (substituted) by a halogen atom.
When an alkane has 3 or more carbons, different products can be formed. Give an example of this.
Bromine + propane (3 carbon) can make either of these products depending on which carbon the bromine attaches to. Both are structural isomers of each other.
https://i.pinimg.com/736x/3e/d6/1b/3ed61b361f5cb0cb3294b3ff9554ea95.jpg
What do halogen mono-substitutions often result in?
Lots of different isomers, particularly for the long carbon chain alkanes.
Does it matter if you join the halogen (chlorine, bromine, iodine, fluorine) to the top or the bottom when drawing a displayed formula?
No. It doesn’t matter if you join the halogen to the top or the bottom. Either is fine.
REVISION SHEET 2: ALKENES.
REVISION SHEET 2: NO LONGER DISCUSSING ALKANES. WE ARE NOW DISCUSSING ALKENES. A L K E N E S.
How are alkenes produced?
Alkenes are produced by cracking large alkanes.
What are alkenes used for?
Alkenes are fuels but are used more as plastics.
Are alkenes saturated or unsaturated hydrocarbons?
They’re unsaturated hydrocarbons because they have a double carbon bond functional group (C=C).
What is the general formula for alkenes?
CnH2n
Alkenes with the following number of carbons are called what?
a) 2
b) 3
c) 4
d) 5
a) Ethene (2)
b) Propene (3)
c) Butene (4)
d) Pentene (5)
Why does methene not exist?
Methene doesn’t exist because all alkenes have a C=C bond. The meth- prefix means 1 carbon.
What is the molecular formula for the following alkenes:
a) Ethene
b) Propene
c) Butene
d) Pentene
a) C2H4
b) C3H6
c) C4H8
d) C5H10
Give the structural formulas for the following alkenes:
a) Ethene
b) Propene
c) Butene
d) Pentene
a) CH2=CH2
b) CH2=CHCH3
c) CH2=CHCH2CH3
d) CH2=CHCH2CH2CH3
Draw the displayed formulas for the following alkenes:
a) Ethene
b) Propene
c) Butene
d) Pentene
a) https://i.pinimg.com/736x/47/f7/2b/47f72bd5e01ba9af8b6c049809c3a213.jpg
b) https://i.pinimg.com/736x/07/ba/06/07ba06bf894f0a67689cab63a03fbfc0.jpg
c) https://i.pinimg.com/736x/17/cf/a6/17cfa6b61faa8eb9f74f1eb8f4e50968.jpg
d) https://i.pinimg.com/736x/2e/c1/2a/2ec12a21d686df0569b3d4b424aa01d6.jpg
Small alkanes and alkenes are what state at room temperature?
They are gases at room temperature.
Small alkanes and alkenes are what state above room temperature?
Liquid, to around C16H32, then they are solid.
What is ethene commonly called?
Ethylene.
What are the properties of ethylene/ethene that make it considered the most useful alkene?
Being a colourless gas, having a faint sweet smell, having about the same density as air, burning with a yellow flame, being insoluble in water, and turning orange bromine water colourless.
How can you find out which alkene an isomer is of?
Count the number of carbons in the main chain. It must have a C=C bond.
Which alkenes are these structural isomers of? Can you name them?
https://i.pinimg.com/736x/d3/39/13/d33913ba85a86ee158a3c67d7e923f3b.jpg
The first one is butene and the second one is propene. Their names are:
1. But-2-ene
2. 2-methylpropene
Are alkenes more or less reactive than alkanes? Why?
Alkenes are MORE reactive than alkanes due to the C=C bond.
What two alkene reactions must you learn?
Combustion AND addition.
Explain the combustion of alkenes.
Alkenes will combust like any hydrocarbon but are rarely used as fuels. Plastics are more useful.
C4H8 + 6O2 –> 4CO2 + 4H2O
(This is butene, as there are 4 carbons)
Explain the addition reaction involving alkenes. How does it work? Why is it called the addition reaction? What does it produce?
Alkenes react with halogens to make haloalkanes. This is called an addition reaction because the halogen is added directly to the alkene. There’s no substitution. There is one product. A haloalkane. The halogen is able to add directly to the alkene because of the alkenes double bond. We can say that the addition reaction is possible because the alkene is unsaturated.
The C=C bond is 2 covalent bonds (4 shared electrons). The double bond ‘opens up’, leaving a single covalent bond linking the 2 carbons. The now spare 2 electrons can form single bonds with other atoms.
Alkanes and alkenes both make haloalkanes. True or false?
True.
Give an example of the addition of an alkene and a halogen.
The addition of bromine (the halogen) and ethene.
The double bond ‘opens up’. Bromine water is orange so the reactants will be orange.
Each carbon can then bond to a halogen.
We know the reaction has occurred when a colourless liquid (1,2-dibromoethane) is produced. 1,2-dibromoethane is the haloalkane.
https://i.pinimg.com/736x/8c/e2/7a/8ce27abb0e7617240d0dad67141e5814.jpg
What is the four step method for naming haloalkanes in addition reactions?
- Count the number of halogen atoms.
- What is the halogen?
- What was the original alkene? Make it an alkane.
- What carbon atoms are the halogens on?
Now, apply this four step method of naming haloalkanes to name the haloalkane produced during the reaction between bromine gas and ethene.
- Count the number of halogen atoms: di (2)
- What is the halogen: dibromo
- What was the original alkene? Make it an alkane: dibromoethane
- What carbon atoms are the halogens on: 1,2-dibromoethane
When naming haloalkanes in addition reactions, we use the halogen nicknames after stating the number of halogen atoms (using di, tri, tetra, penta, hexa, hepta, and octa). What are the halogen nicknames?
Bromine –> Bromo
Chlorine –> Chloro
Fluorine –> Fluoro
Iodine –> Iodo
When naming haloalkanes in addition reactions, we start by stating the number of halogen atoms. However, we don’t use numbers. What do we say to represent the numbers 2, 3, 4, 5, 6, 7, and 8?
2 - di
3 - tri
4 - tetra
5 - penta
6 - hexa
7 - hepta
8 - octa
To test your ability to draw displayed formula, draw the displayed formula EQUATION for propene and bromine and state the name of the haloalkane formed.
Propene = C3H6
Bromine = Br2
https://i.pinimg.com/736x/a2/0b/9f/a20b9f6f714ec21c45d79f12dc5b782e.jpg
1,2-dibromopropane
1. di because there are 2 Br atoms
2. bromo because bromine is the halogen used
3. prop because there are 3 carbons
4. ‘ane’ because there are no double bonds after addition
5. ‘1,2’ because the bromine are linked to the 1st and 2nd carbon
What two methods are there to test whether a chemical is an alkene or alkane i.e. has a C=C bond or not?
- A combustion reaction.
- Haloalkane reaction with bromine.
How can we use a combustion reaction to test whether a chemical is an alkene or an alkane?
Alkenes are more likely to incompletely combust so will produce carbon monoxide (CO). Alkanes are more likely to completely combust, so they won’t produce carbon monoxide (CO). This is unreliable (as alkenes can completely combust and alkanes can incompletely combust).
How can we use a haloalkane reaction with bromine to test whether a chemical is an alkene or an alkane?
This is an addition reaction. Mix the unknown liquid (the one we’re testing) with orange bromine water and shake. If the liquid is decolourised, a dibromoalkane has been produced. We then know the substance is an alkene. If the liquid stays orange, the substance is an alkane. If your unknown substance is a gas, bubble the chemical through the bromine water.
An alkane + bromine turns from orange to colourless, but so does an alkene + bromine. The key difference is the bromine in an alkane substitution reaction is GAS. The bromine in an alkene addition reaction is liquid. The haloalkane in this reaction will always be dibromoalkane because the alkene gains two bromines, not one.
Alkene + bromine water (orange) –> dibromoalkane (colourless)
Alkane will not react with bromine water, so will remain orange.
Alkene WILL react with bromine water, so will go colourless.
