Organic synthesis

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Emily

Cards (34)

Preparing an organic liquid
Quickfit apparatus:
to carry out an organic preparation you will need to use quickfit apparatus - contains:
round-bottom or pear-shaped flask
receiver
screw-tap adaptor
condenser
still head
Heating under reflux:
many organic reactions occur slowly at room temp
common for organic reactions to be heated to overcome the activation energy and increase the rate of reaction
heating under reflux is a common procedure used to prepare an organic liquid without boiling off the solvent, reactants or products
to heat under reflux - need following equipment:
round-bottom or pear-shaped flask
condenser
rubber tubing
stand and clamp
heat source (usually Bunsen burner, tripos and gauze or a heating mantle)
a water bath can be used if the reaction can be carried out below 100 degrees
using a Bunsen burner allows a reaction under reflux to be carried out at a fixed temperature
for heating flammable liquids a heating mantle can be used so that there is no naked flame present. this provides an added level of safety should any of the apparatus leak or crack
clamp the flask by its neck
before fitting the condenser need to add the reaction mixture and anti-bumping granules to the flask
anti-bumping granule are added to the liquid before the flask is heated so that the contents will boil smoothly - if the granules are not used, large bubbles form at the bottom of the liquid and make glassware vibrate or jump violently
finally add a thin layer of grease to the ground-glass joint on the condenser
place the condenser carefully into the flask and gently rotate the condenser back and forth to provide a good seal and ensure the apparatus comes apart easily at the end of the experiment
during reflux the condenser is kept in the upright position
condensers should only be clamped loosely as the glass outer jacket is fragile and easily broken
never put a stopper in the top of the condenser otherwise you would have a closed system and pressure would build up inside as the heated air expanded - apparatus exploding
rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink
water always enters the condenser at the bottom and leaves at the top to ensure that the outer jacket is full
heating under reflux enables a liquid to be continually boiled whilst the reaction takes place
prevents volatile components from escaping and the flask from boiling dry
the vapour from the mixture rises up the inner tube of the condenser until it meets the outer jacket containing cold water
the vapour then condenses and drips back into the flask
similar to putting a lid on a sauce pan when cooking
Distillation:
chemical reactions may not go to completion or may produce by-products as well as the desired products
once reflux is complete, the crude liquid or solid product in the flask needs to be purified to remove any by-products and remaining reactants
distillation is a common method used to separate a pure liquid from its impurities
to carry out a distillation need:
round-bottom or pear-shaped flask
condenser
rubber tubing
heat source
stand and clamp
screw-cap adaptor
receiver adaptor
still head
thermometer
flask clamped by its neck and still head is connected to the flask
still-head adaptor is T shaped and has 2 ground-glass joints, one to fit the screw-cap adaptor and one to fit the condenser
grease the joints so that the apparatus comes apart easily after the experiment
second clamp is placed round the receiver adaptor at the point at which it is attached to the condenser
this removes the need to clamp the condenser as it will be sufficiently supported at both ends
as with reflux, rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink
water always enters the condenser at the lowest point
for distillation - this is the closest point to the receiver adaptor
a flask is used to collect the distillate so that the distillation apparatus is not completely airtight
once the apparatus is set up the flask is heated and the mixture in the flask will start to boil
the different liquids in the mixture will have different boiling points
the liquid with the lowest boiling point is the most volatile and will boil first
the vapour moves up out of the flask up into the other parts of the apparatus, leaving behind the less volatile components of the mixture
when the vapours reach the cold condenser, the condense and become a liquid
this liquid then drips into the collecting flask
Purifying organic products:
when preparing sample of organic liquids water may be obtained along with the product
if this happens will see 2 liquid layers inside collection flask
one is the organic layer and one the aqueous or water layer
easy to separate these 2 although important to know which is the organic layer so don't throw away product by mistake
easy way to identify is to add some water to mixture - layer that gets bigger is the aqueous layer
once organic layer has been identified - 2 layers are separated using a separating funnel:
ensure tap of separating funnel is closed
pour mixture of liquids into separating funnel, place stopper in top of funnel and invert to mix contents
allow layers to settle
add some water - see which layer increases in volume - aqueous layer
place conical flask under separating funnel, remove stopper and open tap until whole of the lower layer has left funnel
place second conical flask under sep fun to collect other layer
one conical flask = organic one = aqueous - label
in preparations using acids - impure product may contain acid impurities - can be removed by adding aqueous sodium carbonate and shaking the mixture in the sep fun
any acid present will react with the sodium carbonate releasing CO2 gas - tap needs to be slowly opened, holding stoppered separating funnel upside down to release any gas pressure that may build up
finally the aq sodium carbonate layer is removed and the organic layer washed with water before running both layers off into 2 separate flasks
Drying the organic product:
may be some water left in the organic product
traces of water are removed by adding a drying agent to the organic liquid
a drying agent is an anhydrous inorganic slat that readily takes up water to become hydrated
anhydrous calcium chloride - CaCl2 - drying hydrocarbons
anhydrous calcium sulfate - CaSO4 - general drying
anhydrous magnesium sulfate - MgSO4 - general drying
procedure for drying an organic liquid:
add the organic liquid to a conical flask
using a spatula, add some of the drying agent to the liquid and gently swirl the contents to mix together
place a stopper of the flask to prevent your product evaporating away - leave for 10 min
if the solid has all stuck together in a lump, there is still some water present - add more drying agent until some solid is dispersed in the solution as a fine powder
decant the liquid from the solid into another flask. if the liquid is dry it should be clear
Redistillation:
sometimes organic liquids have boiling points that are relatively close together - prepared sample may still contain some organic impurities
distillation apparatus is cleaned and dried and set up again so that a second distillation can be carried out
this time, only collect the product with the boiling point of the compound you are trying to make
the narrower the boiling point range, the purer the product
will now have separated the product from any impurities
organic synthesis is the preparation of complex molecules from simple starting materials:
with it chemists can make entirely new structures that can be investigated for their uses
the manufacture of modern medicines targeted at diseases owes much to the work of organic chemists
many chemists are employed by large pharmaceutical companies to synthesise and test new medicines e.g. those designed to target the growth of cancer cells in human body
the term target molecule is used to describe the compound that the chemist is attempting to prepare by organic synthesis
in a simple synthesis - the target molecule can be obtained by reacting a readily available starting material with a readily available reagent in a one-step reaction
in most cases synthesis is not that straightforward:
some organic syntheses require many steps to change functional groups or to add carbon atoms to the chain length in order to obtain the target molecule
some people believe that the natural resources of our planet provide cures for every disease or illness and only time and the limits of our knowledge prevents us from finding a cure for every illness
bark of willow tree has been used for centuries for the treatment of fever and pain
salicylic acid was found to be the active component in willow bark, form which aspirin was developed
as far back as 400BC willow leaf tea was used to relive the pain of childbirth
just 50 years ago, paclitaxel was obtained from the bark of the Pacific yew tree
Taxol is currently used as an anti-cancer drug specifically for the treatment of ovarian, breast and colon cancer
demand for Taxol is much higher than the small amounts that can be obtained from yew bark
to meet this demand chemists synthesise Taxol from more readily available substances