(YELLOW) halogenoalkanes Flashcards
(YELLOW) halogenoalkanes (41 cards)
describe the solubility of halogenoalkanes
However, halogenoalkane molecules are not polar enough to make even those molecules with the shortest chains soluble in water. There are no Hd+ atoms so hydrogen bonding with water cannot occur. They are immiscible in water. They will dissolve in organic solvents such as alcohols and ethers.
what kind of intermolecular forces are present
london forces
perminent dipole - perminent dipole
What is the trend in boiling point with increased chain length?
Explain your answer
increases
Increased chain length —> more electrons —> stronger London forces.
Require more heat energy to overcome them
What is the trend in boiling point as you go down the halogen group?
Explain your answer
increases
Increased number of electrons —> stronger London forces.
Require more heat energy to overcome them
What is the trend in boiling points 1o –> 2o –> 3o
Explain your answer.
decreases
Increased branching —> molecules further apart —>weaker London forces. (Also branched molecule —-> smaller dipole —> weaker London forces.)
So less heat energy required to overcome them and separate the molecules —-> vapour
how do you go from CH3CHOHCH3 to CH3CHClCH3
give reagents and conditions
This is not a very economical way to make large quantities as PCl5 is expensive, can you think of another reason?
Reagents phosphorus pentachloride Conditions room temp
CH3CHOHCH3 + PCl5 —> CH3CHClCH3 + POCl3(l) + HCl(g)
Hydrogen chloride gas produced is corrosive
could also use HCl and it would produce water instead
how do you get a bromoalkane from an alcohol
give reagents and conditions
also give the reaction need to make PBr3
1) From Bromine and moist red phosphorus (forming PBr3 in situ)Equation to make PBr3 :
P + 1½Br2 —> PBr3
3x(secondary alcoho)l + PBr —> 3 x(secondary halogenoalkane) + H3PO3
how do you get a iodoalkane from an alcohol
give the conditions and reagents
These can only be made from Iodine and red phosphorus (PI3 made ‘in situ’)
Equation: P + 1½I2 —-> PI3
conditions warmed + moist
3 x (secondary alcohol) + PI3 —> iodoalkane + H3PO3
what is the aparatus use to connect the tap funnel to the condencer
distillation head
what is the aparatus that is used to add the conc sulfuric acid in
tap funnel as its too corrosive to handle drop wise
Write an equation for the reaction of concentrated sulphuric acid with sodium bromide.
NaBr(s) + H2SO4(50% conc) —-> NaHSO4(aq) + HBr(g)
Why is it necessary to add the concentrated sulphuric acid slowly and cool the flask in cold water?
To avoid the reaction of 2HBr + H2SO4 —> SO2(g) + Br2(g) + 2H2O(l)
HBr acts as a reducing agent reducing H2SO4 to SO2 and being itself oxidised to Br2
Why is there a tube with an open-side arm between the condenser and the tap funnel?
So it is not a closed system where pressure of vapour could build up.
What changes did you see in the flask as you added the acid and then heated the mixture?
Steamy fumes (and orange fumes)
Can you use your observations to identify any of the products formed?
Steamy fumes of HBr, possibly SO2 and orange fumes of Br2
Explain what is meant by heating under reflux. Why is this often necessary when heating organic liquids?
Heated solution evaporates and vapour rises up vertical condenser.
Vapour condenses and drops back into the reaction vessel.
This avoids flammable vapours being released
It allows a longer reaction time with no loss of flask contents.
What is the reason for including a guard tube in the apparatus during the refluxing stage?
Contains soda-lime which reacts with any acidic gases (HBr) preventing their release
Why is it necessary to heat the flask for such a long time?
To ensure complete reaction.
can H2SO4 dissolve in water
yes
info card read and recite
Purifying the product
The impure product is shaken with conc.HCl.Butan-1-ol dissolves in the conc. HCl.
1)Why are there two layers in the separating funnel when the product is shaken with aqueous reagents?
Bromobutane is immiscible in water/aqueous solutions as it only has permanent dipole forces and is unable to break up the stronger H-bonding between water molecules.
2) Suggest a reason why shaking the product with hydrochloric acid helps to remove unchanged butan-1-ol from the impure product.
Butan-1-ol is soluble in HCl because it does H-bonding
Conc. HCl protonates the butan-1-ol —> CH3CH2CH2CH2OH2+, which is ionic and therefore soluble
3) Which impurities are removed by shaking the product with a solution of sodium hydrogencarbonate? Acidic impurities e.g. original H2SO4, HBr from reaction, HCl added
Why is there a marked build up of pressure during the shaking? CO2 produced
Write an ionic equation for this reaction H+(aq) + HCO3-(aq) —> H2O(l) + CO2(g)
4) Suggest a reason why sodium hydrogencarbonate solution is used rather than sodium hydroxide solution. :OH- would act as a nucleophile substituting for Br —> butan-1-ol
5) How did the appearance of your product change when it was swirled with anhydrous sodium sulphate? Cloudy liquid (emulsion) became a clear colourless liquid.
How do you account for this?
Removal of water
6) Name another drying agent which could be used in place of the anhydrous sodium sulphate.
Anhydrous salt eg. CuSO4 , MgSO4, CaCl2, or silica gel crystals
info card read and recite
All the impurities should now have been removed, except small amounts of organic impurities which will be dissolved in the bromobutane. These can be separated using distillation.
At first, you will see organic impurities coming out of solution, but nothing will condense in the receiver. Then the temperature of the vapour will rise.
The boiling point of 1-bromobutane is 102°C, we collect the fraction which distills over in the range 100 °C to 104°C.
Higher boiling point impurities will remain in the flask
Explain the term ‘fraction’ to describe the sample of product collected during the final distillation.
Part of the whole sample having a narrow specified boiling point range
How does the final stage allow you to check the identity and purity of your product?
Should all distil over at 102oC, any impurities will lower the B.Pt.
Experience suggests that you will do well to get a yield over 50 %.
What reasons can you think of to account for a yield well under 100 %?
Reaction may not have gone to completion, side reactions produce other products, lengthy separation and purification process result in losses during transfer.
