halogenoalkanes Flashcards
What is electronegtivity?
- a measure of the power of an atom to attract the shared pair of electrons in a covalent bond.
What is the most electronegative element?
- flourine
What determines electronegativity?
- nuclear charge
- atomic radius
- sheilding
What is the trend in electronegativity in a group?
Electronegativity decreases down a group
- greater atomic radius
- more sheliding
What is the trend in electronegativity across a period?
- electronegativity increases
-same sheliding - greater nuclear charge
- smaller atomic radius
What is polarity?
- the unequal sharing of electrons between atoms that are covalently bonded
- bond polarity arises from covalent bond between atoms with different electronegativity
What do we refer to the charge separation as?
- dipole
How do we name halogenoalkanes?
- use the prefix e.g. fluoro then the name of the original alkane is taken into account
The position of the halogen is taken into account and is named alphabetically.
What is a primary halogenoalkne?
- when a halogen is attached to a carbon that is itself attached to another akyl group
What is a secondary halogenoalkane?
- when a halogen is attached to a carbon that itself is attached to 2 other akyl groups
What is a tertiary halogenoalkane?
- when a halogen is attached to a carbon that itself is attached to three other alkyl groups
What is the trend in the strength of the carbon- halogen bond down the group?
- decreases down the group C-F is the strongest
During a substitution reaction, how does a C-I bond break?
- Because it is the weakest bond out of the c-halogens, it will break heterolytically during a substitution reaction.
What is the equation for the substitution reaction between C-I?
- R3C-I +OH- -> R3C-OH + I-
During a substitution reaction, how does a C- F bond break?
- The C_F bond requires the most energy to break and is therefore the strongest carbon-halogen bond
- Fluoroalkanes are then less likely to undergo substitution reactions.
What happens when you react aqueous silver nitrate with a halogenoalkane?
- results in the formation of a precipitate
- The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes
What colour precipitates are formed when aqueous silver nitrate is added?
- chlorides - white (silver chloride)
- bromides - cream (silver bromide)
- iodides - pale yellow (silver iodide)
Why is the formation of silver iodide the quickest?
- because it has the fastest nucleophillic substitution between iodine and nitrous
How do you measure the rate of hydrolysis of the halogenoalkanes?
- Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
- Set up three test tubes in a 50 C water bath with a mixture of ethanol and acidified silver nitrate
- Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a
stop watch - Time how long it takes for the precipitates to form
- The precipitate will form as the reaction progresses and the halide ions are formed
- A white precipitate will form from the chloroalkane, a cream precipitate will form from the
Bromoalkane and a yellow precipitate will form from the iodoalkane
What is a nucleophile?
- A nucleophile is an electron-rich species that can donate a pair of electrons.
Why is a hydroxide a better nucleophile than water?
- Because it has a full formal negative charge, whereas the oxygen atom in water only carries a partial negative charge.
What are the typical nucleophiles?
- OH- -hydroxide ion
- CN- Cyanide ion
- NH3 - Ammonia
These all have lone pairs
What is a nucleophilic substitution reaction?
- A nucleophillic substitution reaction is one in which a nucleophile attacks a carbon which has a partial positive charge
- an atom that has a partial negative charge is replaced by the nucleophile
Why do halogenoalkanes undergo nucleophilic substitution reactions?
- due to the polar C-Halogen bond where there are large differences in electronegativity between the carbon and halogen atom.
What is the general mechanism for nucleophilic substitution?
- the nucleophile goes to the carbon atom while the elctrons are shifted to the halogen this forms an alakne with the nucleophile on the end and the halogen that is now an ion with lone pairs.
What happens in nucleophilic substitution with cyanide ions?
- The ethanolic solution of potassium cyanide is heated under reflux with the halogenoalkane.
-The product is a nitrile
This adds an extra carbon atom to the carbon chain
What can the reaction of KCL with a halogenoalkane be used for?
- used to make a compound with more than one carbon atom than the best available organic starting material
What happens in the nucleophilic substitution with ammonia?
- an ethanolic solution of excess ammonia is heated under pressure with the halogenoalkane’
- the product is a primary amine
What are the conditions needed for substitution reactions?
- NaOH, OH- needs to be aqueous and warm
- KCN, CN- - needs to be ethanolic and warm
- NH3 needs to be in excess, and concentrated ammonia needs to be dissolved in ethanol at pressure in a sealed container
How does an elimination reaction occur?
- Nucleophiles have lone pairs of electrons. This means they can form diative covalent bonds with H+; they are bases . Warm, aqueous sodium hydroxide acts as a nucleophile.
Nucleophilic substitution reactions occur with halogenolakanes. This forms an alcohol. If we change the conditions of the reaction, we can change the products. Under different conditions, OH- will act as a base. This will remove H+ from the halogenoalkane. This is an elimination reaction rather than substitution
What are the conditions for an elimination reaction?
- heat
- Ethanol as a solvent
- Concentrated potassium hydroxide
What happens in an elimination reaction?
- the halogenoalkanes are heated with ethanolic sodium hydroxide, causing the carbon-halogen bond to break heterolytically,y forming a halohen- ion and leaving an alkene as an organic product
What is the mechanism for an elimination reaction?
- The electrons move to the halogen to form a partially negative charge on the halogen and a partially positive charge on the carbon it is attached to.
-the nucleophile OH- will take a Hydrogen from the carbon - this forms an alkene, water and a halogen ion.
What are the differences in conditions between substitution and elimination?
- Elimination has a high temperature substutution has a low temperature
- elimination has a concentrated amount of hydroxide, substitution is dilute.
- The solvent in elimination is ethanol, water in substitution.
- Hydroxide acts as a base in elimination and acts as a nucleophile in substitution.
