topic 3: halogenoalkanes Flashcards
what is a nucleophile and name 3 examples
a nucleophile is an electron pair donor to an electron deficient carbon atom, it is a negatively charged ion with a lone pair of electrons which it can use to form a covalent bond
e.g. -hydroxide ion: :OH-, cyanide ion: :CN-, ammonia molecule: NH3
bonding in halogenoalkanes (the carbon-halogen bond)
-halogenoalkanes C-X contain polar bonds
-halogens are more electronegative (d-) than carbon (d+)
-the d+ on the carbon atom makes it prone to nucleophile attack, because the nucleophile likes regions of positivity
-as you go down group 7 electronegativity decreases, this means the bond polarity decreases
-there are Van Der Waals and dipole dipole attractions
explain why the carbon-halogen bond enthalpy influences the rate of reaction - trends with reactivity
use of examples: fluoroalkanes and iodoalkanes
-the CARBON-HALOGEN BOND ENTHALPY STRENGTH determines REACTIVITY, for any reaction to occur the carbon-halogen bond needs to BREAK
-reactivity increases down the group, faster substitution as bond enthalpy decreases
-this is due to weaker bonding between carbon and the halogen
-the bigger the halogen atom, the further away the nucleus is to the outer electron, so less attraction, weaker bond - this is due to bond enthalpies
-the C-F bond is the STRONGEST, it has the highest BOND ENTHALPY , fluoroalkanes undergo nucleophillic substitution reactions MORE SLOWLY than other halogenoalkanes
-the C-I BOND has the LOWEST BOND ENTHALPY, so it is easier to break, therefore IODOALKANES are substituted more QUICKLY
rate of reaction in nucleophillic substitution
-the rate of this reaction increases as the C-X bond decreases in strength (down group)
HYDROXIDE ION
-reagent used in nucleophilic substitution?
-conditions used in nucleophilic substitution?
-what product forms in hydroxide ion in nucleophillic substitution?
-overall equation in hydroxide ion of nucleophilic substitution
- sodium hydroxide (NaOH) to get :OH- hydroxide ion
- aqueous and warm
- halogen atom is replaced by OH group to form an alcohol
- R-X + NaOH —-> R-OH + NaX
CYANIDE ION
-reagent used in nucleophilic substitution?
-conditions used in nucleophilic substitution?
-what product forms in cyanide ion in nucleophillic substitution?
-overall equation in cyanide ion of nucleophilic substitution
- potassium cyanide (KCN) to get :CN- (cyanide ion)
- ethanolic and warm
- halogen group is replaced by the CN group to form a nitrile
- C-X + KCN —-> C-CN + KX
AMMONIA MOLECULE
-reagent used in nucleophilic substitution?
-conditions used in nucleophilic substitution?
-what product forms in ammonia molecule in nucleophillic substitution and what does it act as?
-overall equation in ammonia molecule of nucleophilic substitution
- NH3 need 2NH3
- excess ammonia dissolved in ethannol at pressure in a sealed container
- halogen atom is replaced by NH3 group from first NH3 group, NH3 acts as a nucleophile
-second molecule of NH3 is used to make NH4X (X is halogen), it acts as a BASE (proton acceptor - H)
-the end product is an amine - C-X + 2NH3 —-> C-NH2 +NH4X
what does an elimination reaction eliminate and form?
-eliminates a hydrogen and halogen
-forms a double bond, ALKENE
how does a hydroxide ion act in an elimination reaction and a nucleophillic substitution reaction?
elimination reaction: BASE - acts as a proton acceptor (H+), by removing a H+ ion from the halogenoalkane, without replacing anything forming alkenes (C=C)
substitution reaction: NUCLEOPHILE - electron pair donor, to an electron deficient carbon atom, replacing halogen in a halogenoalkane
conditions used with a hydroxide ion in a substitution and elimination reaction
CONDITIONS IN A SUBSTITUTION REACTION
-room temperature, warm and aqueous solutions
CONDITIONS IN A ELIMINATION REACTION
-high temperature, hot and anhydrous conditions
-reflux reaction
-uses ethanolic solutions
HYDROXIDE ION
-reagent used in elimination?
-conditions used in elimination?
-what happens in hydroxide ion in elimination?
-overall equation in hydroxide ion of elimination
- potassium hydroxide (KOH) for the :OH-
- conditions: high temperature, hot and ethanolic solution
- what happens: halogen atom and hydrogen atom is from adjacent carbon is removed to form an alkene and water.
- ___ + KOH → ___ + H2O+ KX
what is the ozone layer and how is it beneficial?
-the ozone, formed naturally in the upper atmosphere
-ozone is beneficial because it absorbs ultraviolet radiation and prevents harmful radiation reaching the earth, protecting living organisms from excessive UV exposure
ozone formation
-UV radiation causes the double bond in oxygen molecules to break:
-oxygen molecule breaks down into 2 oxygen radicals O2 —-> 2O.
-an oxygen radical then reacts with an oxygen molecule and forms ozone, this is a reversible reaction:
O. + O2 <===> O3
+UV
what are CFCs (chloroflurocarbons) and their purpose?
-halogenoalkanes, chemicals containing chlorine, fluorine and carbon which contribute to ozone depletion
properties of CFCs and uses
-they are pretty STABLE, unreactive and NON-TOXIC due to the high enthalpy of the C-halogen bond and are NON-FLAMMABLE and used to be used as:
-refrigerants (cooing gas in fridges)
-solvents
why are CFCs banned?
-In 1970s, research by several different scientific groups demonstrated that CFCs were causing damage to the ozone layer
-the advantages of CFCs were outweighed by the environmental problems so they are banned for use in refrigerants and solvents
-therefore alternatives were developed which contain no chlorine such as HFCs (hydrofluorocarbons) and hydrocarbons, which are safer
what atoms are formed in the upper atmosphere and how?
-chlorine atoms are formed in the upper atmosphere when UV radiation causes C-Cl bonds in chlorofluorocarbons (CFCS) to break
oozone depletion: what do chlorine atoms catalyse and contribute to?
-chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer
-C-Cl bonds in CFCs are broken down by UV and form chlorine free radicals upper atmoshpere
-these free radicals are catalysts
-one CFC molecule can lead to the destruction of many O3 molecules
equation for chlorine atoms catalysing the decomposition of ozone
-they react with ozone to form an intermediate (ClO.) and an oxygen molecule
-Cl. + O3 —-> ClO. + O2
-ClO. + O3 —-> 2O2 + Cl.
-so the overall equation is 2O3 —> 3O2 and Cl. is the catalyst
