DF6 - Alkenes Flashcards
What do unsaturated fats contain?
C=C bonds + are related to alkenes
Which compounds are found to be very likely to lead up to a build-up of cholesterol? Cis or trans?
Trans fats
How are alkenes distinguished from other hydrocarbons?
By the presence of a C=C double bond which implies they are unsaturated hydrocarbons
Does the boiling points of alkenes increase or decrease as the number of C atoms increases?
The bp increases - just like alkanes
What is the general formula of alkenes?
CnH2n
How are the rules for naming alkenes different from naming alkanes?
The ending of alkenes is ‘ene’
Everything else is the same
What the other unsaturated hydrocarbons?
- Cycloalkenes
* Dienes
What is a diene?
An unsaturated hydrocarbon containing 2 C=C double bonds
How many C=C double bonds do cycloalkenes have?
One C=C double bond
What is a sigma bond?
A direct overlap of s-orbitals (sigma orbitals) and an area of increased electron density
What does a C-C bond consist of?
The two electrons are arranged between atoms in an area of increased electron density called a sigma bond
What does a C=C double bond consist of?
A sigma bond + a pi bond
What is a pi bond?
One pi bond consists of two areas of negative charge, one above + one below the line of atoms
Draw a diagram to represent the sigma + pi bonds in a C=C double bond
Sigma bond is in between the two areas of negative charge which make up the pi bond. The sigma bond is also in between the two C atoms in C=C double bond)
What is an electrophile?
A positive ion, or molecule with a partial positive charge, that will be attracted to a negatively charged region + react by accepting a lone pair of electrons to form a covalent bond
Describe and explain the double bond in ethene. What is attracted to the C=C bond and what will happen when they react?
- The 4 electrons in the double bond of ethene give the region between the the two C atoms a higher than normal density of negative charge
- Electrophiles will be attracted to this negatively charged region + they can react by accepting a pair of electrons from the C=C double bond
What happens when ethene gas is bubbled through bromine? What is it a good test for?
The red-brown bromine becomes decolourised - this is a good general test for unsaturation in an organic compound
What does lower case delta mean?
(In this case) small charge
What is a dipole?
A molecule that has two different charges at opposite ends
What does the curly arrow show?
Shows two electrons moving from one area to another
What does it mean for a molecule to be polarised?
To have a balance of charges (neutral) but one side is negative and one side is positive (neg. + pos. charge in centre splits to opposite sides)
What is a carbocation?
An ion with a positively-charged carbon atom.
Carbocations react very rapidly with anything that has electrons to share
What is a mechanism?
An organic reaction mechanism represents the sequence of events in a reaction using the movement of electrons represented by curly arrows
What is an addition reaction?
A reaction where two or more molecules react to form a single, larger molecule
What is electrophilic addition?
An addition reaction involving the electrophile initially attacking
What is the other test for alkenes? Explain how it works
- Shaking the alkene with bromine water
- Water molecules have lone pairs of electrons + can act as nucleophiles in competition with bromide ions
- If bromine water is dilute, there will be many more water molecules than bromide ions present + the bromoalcohol will be the main product of the reaction
- This does not affect what you see - the bromine water is still decolourised
Describe the reaction of ethene with hydrogen bromide + draw the reaction with product
- Ethene reacts readily at room temp. with a solution of HBr in a polar solvent - this is another example of electrophilic addition
- Alkenes also react with gaseous HBr but ions are not involved + the mechanism involves a radical addition
Describe ethene’s reaction with water + draw the reaction with product
- At a high temp. + in the presence of a catalyst (phosphoric acid adsorbed onto solid silica), ethene and water (as steam) undergo an addition reaction
- The process is used for the industrial manufacture of ethanol
- The overall reaction is addition of water across the double bond. The addition of water to an alkene is an example of a hydration reaction
Describe how ethene can be converted to ethanol in a laboratory + draw the mechanism for this
- In the laboratory, ethene can be converted to ethanol by first adding concentrated sulfuric acid + then diluting with water
Describe the reaction between ethene and hydrogen
- It is another addition reaction but it involves H atoms + takes place on the surface of the catalyst
- A catalyst is needed to help break the strong H-H bond + form H atoms that can react with the alkene
- If a platinum catalyst is used, the process takes place under standard lab. conditions
- Nickel is a cheaper but less efficient catalyst - it needs to be finely powdered + the gases needheating to approx. 150 degrees under 5 atm pressure for hydrogenation to occur
- This hydrogenation reaction is the reaction used to make the unsaturated fats + oils more saturated
