(U2) Halogenoalkanes Flashcards
What is a primary halogenoalkane? (2)
What are the exceptions?
- A halogenoalkane which has 1 carbon atom directly bonded
- to the carbon atom that is bonded to the halogen
- halomethanes
What is a secondary halogenoalkane? (2)
- A halogenoalkane which has 2 carbon atoms directly bonded
- to the carbon atom that is bonded to the halogen
What is a tertiary halogenoalkane? (2)
- A halogenoalkane which has 3 carbon atoms directly bonded
- to the carbon atom that is bonded to the halogen
What is reflux?
Repeated boiling and condensing of a reaction mixture
What is hydrolysis? (Chemistry)
Breaking up molecules by reaction with water
What is a nucleophile? (3)
- An ion or molecule,
- with a lone pair of electrons,
- that attacks regions of low electron density
What is elimination?
A reaction in which a small molecule is removed from a larger molecule
What is the general formula of halogenoalkanes?
CnH2n+1X
What is the trend in the boiling point of halogenoalkanes? (3)
- boiling point increases with no. of halogens - greater VdV
- also increases as the size of the halogen atom increases (e.g. fluoroalkanes would have a lower boiling point than chloroalkanes) - greater VdV
- also the longer the carbon chain, the larger the boiling point
[describe the laboratory preparation of a liquid organic compound, such as a halogenoalkane, from the corresponding alcohol]
Preparation of 1-bromobutane
Method:
- 30 cm3 of water, 35 g of powdered sodium bromide and 25 cm3 of butan-1-ol in a 250 cm3 round bottomed flask.
- Fit a tap funnel to the flask via a stillhead.
- 25 cm3 of concentrated sulphuric acid in the tap funnel, and then allow the acid to fall drop wise into the flask, keep contents shaken and cooled in an ice-water bath.
- When the addition is complete, replace tap funnel and stillhead with a reflux water condenser and boil the mixture over a sand-bath for about 45 minutes, shaking the flask often
- Remove the reflux condenser and rearrange apparatus for distillation. Distil off the crude 1-bromobutane (about 30 cm3).
- Shake the distillate with water in a separating funnel, and run off the lower layer of 1-bromobutane; reject the aqueous layer.
- Return the 1-bromobutane to the funnel, add about half its volume of concentrated hydrochloric acid, and shake. Run off and discard the lower layer of acid.
- Shake the 1-bromobutane cautiously with dilute sodium carbonate solution, cautiously releasing the pressure at intervals.
- Run off the lower layer of 1-bromobutane and add some granular anhydrous calcium chloride. Swirl the mixture until the liquid is clear.
- Filter the 1-bromobutane into a clean dry flask, and distil it, collecting the fraction boiling between 101 - 103ºC.
What are the conditions required for the nucleophilic substitution reaction between halogenoalkanes and alkalis?
What are the products?
- alkali must be aqueous - hydroxide ions in aqueous solution
- reacts under reflux with hydroxide ions which come from NaOH or KOH
- an alcohol
- a halide salt
What are the conditions required for the reaction between halogenoalkanes and ammonia?
What are the products?
What type of reaction is this?
- Heat with ethanolic ammonia in a sealed tube
- An amine (func group = (-NH2)) and a hydrogen halide, which can react with ammonia to form an ammonium halide
- Nucleophilic substitution
What are the conditions required for the reaction between halogenoalkanes and cyanide ions?
What are the products?
What is the role of the CN- ion?
What effect does the reaction have on the carbon chain?
- Heated under reflux in an ethanolic solution of potassium or sodium cyanide
- A nitrile (func group = (-C≡N))
- acts as a nucleophile
- increases its length
What are chlorofluorocarbons (CFCs) used for? (2)
- reducing the ozone layer
- therefore allowing more harmful ultraviolet radiation to reach the Earth’s surface
Outline the Sn1 and Sn2 mechanisms of nucleophilic substitution reactions with a halogenoalkane
- Sn1
- rate determining step - 1st step (slow) - breaking of the C-X bond to form the intermediate carbocation (induced polarity)
- 2nd step (fast) - nucleophile attacks the carbocation, forming the product
- Sn2
- the nucleophile performs a backside attack, forcing the leaving group to go, forming the product in 1 step
What type of reaction is the reaction between halogenoalkanes and cyanide ions?
Nucleophilic substitution
What are the conditions required for the elimination reaction between halogenoalkanes and alkalis?
What are the products?
- alkali must be ethanolic (dissolved in ethanol as a solvent)
- reacts under reflux
- An alkene and a hydrogen halide, (with a halide salt being a possible byproduct)
What are the general conditions required for a substitution reaction with halogenoalkanes, with regards to:
- temp
- OH concentration
- solvent
And what is the main organic product?
- low temp
- dilute OH conc
- water as a solvent
- An alcohol
What are the general conditions required for a elimination reaction with a halogenoalkane, with regards to:
- temp
- OH concentration
- solvent
And what is the main organic product?
1.
- high temp
- high OH concentration
- ethanol as a solvent
- Alkene
What is the term given to halogenolakanes that undergo nucleophilic substitution reactions with KOH, KCN and NH3?
Unsymmetrical
Which of iodine, chloro or bromoalkanes are more susceptible to elimination?
Iodo>bromo>chloro
How do you draw the Sn2 mechanism for the reaction between a hydroxide ion with bromoethane?
- Reactants:
- draw bromoethane with a δ+ CH3 and a δ- Br.
- Also :OH-
- arrow to transition state:
- in box brackets and negatively charged.
- Wedge, dashed line and line to CH3, H and H.
- Dotted lines to OH and Br (signifying weak bonds).
- arrow to products:
- OH, C, CH3, H and H all bonded (use dashed, wedge and normal lines to indicate tetrahedral structure
- :Br- separate
How do you draw the Sn1 mechanism for the reaction between a hydroxide ion with 2-bromo-2-methylpropane?
- CH3 δ+ and Br δ-, then C and 2 x CH3
- then CH3 x 3, connected to C+, with a separate :OH-
- products: CH3 x 3 + C + OH. Also a separate :Br-
