6.3 [Halogenalkanes] Flashcards
What are halogenoalkanes?
Halogenoalkanes are a homologous series of compounds with a general formula CnH2n+1X.
In halogenoalkanes hydrogen atom is replaced with a halogen atom.
X represents a halogen atom- Usually bromine or chlorine.
The symbol R is often used in organic chemistry to represent an alkyl (such as methyl or ethyl)
How are Halogenoalkanes classified?
Halogenoalkanes are classified primary, second and tertiary. Depending on the number of alkyl groups joined to the C atom bonded to the halogen atom.
What makes Halogenoalkanes reactive?
Hydrocarbons contain only hydrogen and carbon atoms, which have similar electronegativities so their bonds are non-polar
Halogenoalkanes contain a halogen atom with an electronegativity higher than that of carbon, so the C-X bond is is polar. This bond polarity can be indicated using partial sigma + and sigma -
Down group 7 of the periodic table the electronegativities of halogens decrease from fluorine to iodine to the polarity of the C-X bond also decreases
Carbon atom joined to halogen is always slightly positive or electron deficient part of the molecule and this is what makes halogenoalkanes react as they do.
Carbon atom atoms attract other species called nucleophiles. Nucleophile indicates a species that are attracted to slightly positive or electron-deficient parts of the molecule with a slightly negative atom, but they always use a lone pair of electrons when attacking another species
How do Hydrolysis reaction? occur with Halogenoalkanes?
When a halogenoalkane is added to water a reaction occurs, May take a period of time complete. A water molecule contains polar bonds ,and oxygen atom in the water are attracted to the carbons in Halogenoalkanes.
RX + H20 > ROH + HX
product is an alcohol and as both organic substances are colourless liquids, no colour changes can be seen. C-X bonds break. RX molecule breaks down as well.
What is a hydrolysis reaction?
A hydrolysis reaction is one which water or hydroxide ions replace an atom in a molecule with an OH group
How can the rates of Hydrolysis between primary, secondary and tertiary halogenoalkanes be compared?
Instead of adding water silver nitrate solution is added, then the progress of the reaction can be followed.
Ag + X- > AgX
Means that can tell how quickly the hydrolysis reaction occurs be observing how quickly the precipitate of AgX forms.
What does the comparison of rates of the reactions between Halogenoalkanes involve?
Using ethanol and solvent for the mixture (halogenoalkanes and aqueous silver nitrate do not mix but form different layers)
Controlling variables such as temperature and the concentration and quantity of halogenoalkanes.
Timing the appearance of the precipitate(difficult to do accurately) m
What are the results of the hydrolysis comparing the rates of reactions between halogenoalkanes?
No precipitate if formed by chloroalkane
A faint cream-coloured precipitate in tube containing Bromoalkene
A thicker yellow precipitate in Iodoalkene
Comparisons between halogenoalkanes with the same structure but different halogens
Comparing Halogenoalkanes with different structure but same halogen.
Iodobutane would be more reactive followed by bromobutane and Chlorobutane.
What is the reasons for the different rates of reactions?
Halogenoalkanes with most polar bond would be the fastest to hydrolysed because of the delta + charge on the carbon atom is greatest so the attacking nucleophile should be attracted more strongly.
More important factors to consider like Bond breaking requires energy and the weakest bonds break more easily. C-I is the weakest bonds and C-Cl is the strongest increases up the group.
So under same conditions C-I bonds break forming I- ions and AgI forms. Fluoroalkanes are very unreactive.
How can results be interpreted from different structures?
Tertiary Halogenoalkanes are rapidly more hydrolysed than secondary and primary.
What is the first substitution reaction that Halogenoalkanes undergo?
Halogenoalkane reacts with water to form a hydroxide
What is the second substitution reactions that Halogenoalkanes undergo?
Heating a halogenoalkane with aqueous potassium hydroxide, under reflux makes alcohols. The attacking nucleophile is the OH- ion.
CH3CH2CH2Cl + KOH > CH3CH2CH2OH + KCl
What is the third substitution reactions that Halogenoalkanes undergo?
Heating a Halogenoalkane with potassium cyanide dissolved ethanol under reflux makes nitriles. The attacking nucleophile is the CN- ion. For example conversion of bromoethane into propenonitrile.
Organic products contains one more carbon atom than the starting material it extends carbon chains and synthesise complex compounds.
CH3CH2Br + KCN > CH3CH2CN + KBr
What is the fourth substitution reaction that Halogenoalkanes undergo?
Heating a halogenoalkane with ammonia solution in a tube is a way of making primary amines. The sealed tube is needed because ammino is a gas and otherwise escape from apparatus before it could react. Attacking nucleophile is the NH3 molecule. Converting 1-iodobutane into butylamine.
Organic compound reacts with the inorganic product acid HI to form a sault.
To produce a high yield of the amine the ammonia is used in excess some of this excess ammonia reacts in a second step to produce the ammine.
CH3CH2CH2CH2I + NH3 > CH3CH2CH2CH2NH3 + I-
What is Nucleophilic substitution mechanisms?
In each reaction the attacking species is a nucleophile so the reaction type is described as nucleophilic substitution only need to know mechanisms for primary halogenoalkanes undergoing reactions 2 and 4
What do the mechanisms look like for the second reaction?
Bromoethane and potassium hydroxide. Reaction starts with the donation of a lone pair of electrons from the oxygen of a hydroxide ion to the electron-deficient carbon atom formation of a C-O bond. At the same time the electrons in the C-BR move to the Br atom breaking the C-Br bond this type of reaction known as heterolytic fission.
What do the mechanisms look like for the fourth reaction?
Chloroethane and ammonia. First step involves the donation of the lone pair of electrons from the Nitrogen of an ammonia molecule to the electron deficient carbon atom forming a C-N bond. Electrons in the C-Cl bond move to the Cl atom braking the C-Cl bond.
The second step of the reaction involves another ammonia molecule acting as a base and removing a hydrogen ion from the ion formed in the step.
How do elimination occur in nucleophilic substitution?
Reaction 2 is a substitution reaction, using a different solvent (ethanol instead of water) causes a different reaction to occur. Halogenoalkanes is heated with ethanolic potassium hydroxide the OH- ions act as a base and not as a nucleophile. Base reacts with a hydrogen ion the hydrogen that reacts with the OH- ion is the one attached to a carbon atom next to the C in the C-Br bond
When 2-Bromopropane and ethanolic potassium hydroxide react propene I formed and water and potassium bromide are the other products.. H and Br are removed from the Halogenoalkane not replaced by any other atoms.
What are nitriles and primary amines?
Nitrites are organic compounds containing the C-CN group.
Primary Amines are compounds containing the C-NH2
What is an ethanolic?
An ethanolic solution is one which ethanol is a solvent.
What is an elimination reaction?
Elimination reactions are one which a molecule loses atoms attached to the adjacent carbon atoms forming a C=C double bond.
