HL chemistry internal assessment. Awarded level 5. Research question is: 'What is the effect of atomic radius and halogen electronegativity on the reactivity of monohalogenated methane compounds?'.
An investigation into the effect of atomic radius and halogen electronegativity on the reactivity of
monohalogenated methane compounds using a database.
Candidate code: jjs506
Research Question
What is the effect of atomic radius and halogen electronegativity on the reactivity of monohalogenated methane
compounds?
Introduction
Throughout history, halogenated compounds have been utilised as drugs in the medical field. An early example is
chloroform, otherwise known as trichloromethane, which was used as an anaesthetic in the 19th century 1. Previously,
ethers were used as anaesthetics. The key mechanisms of actions of these polyhalogenated anaesthetics involve the
functional modulation of ionic channels and proteins on CNS implicated in the anaesthetic response 2. In the 1940s,
chloroquine (C18H26ClN3 ) 3, a synthetic halogenated organic compound utilized for the treatment of malaria, was
introduced into medicine. It is effective against susceptible strains of the malarial parasites P. Vivax, P. Ovale, and P.
Falciparum. It is a member of an important series of chemically related antimalarial agents, the quinoline derivatives 4,
and is classified as a 4-aminoquinoline3; the quinoline is substituted at position 4 by a [5-(diethylamino)pentan-2-yl]
amino group at position 7 by chlorine3. Currently, there are ongoing studies that research the effect of halogen
substitution on the reactivity of certain anticancer compounds, such as antitumor 3-formylchromones and their free
radicals5. Evidently, halogenated drugs are very versatile, which is due to halogen properties. These favourable properties
include their polarisability, electronegativity, and the intermolecular interactions which contribute to the stability of
protein-ligand complexes6. Fluorine drugs dominate the industry6, whilst chlorine ones are second-most common.
Bromine drugs are rare, and iodine drugs are even rarer; one example includes thyroxine, the thyroid hormone 7. This
pattern is due to the physical and chemical properties of halogens. For example, the highly electronegative nature of
fluorine renders it a poor halogen bond acceptor character, however, it enables it to receive hydrogen bonds from H-bond
donors. This implies that these chemical characteristics can alter physical, chemical, electronic parameters, which could
eventually result in drugs with optimal pharmacological properties 6.
This IA aims to investigate how the atomic radius and halogen electronegativity affect the reactivity of monohalogenated
methane compounds with the general formula CH 3X.
Background Information
Alkanes
Alkanes are saturated hydrocarbons with the general formula of C nH2n+2, and the suffix ‘ane’. These include methane,
ethane, propane, butane, and so on. Some of the general properties of alkanes are listed below:
➔ Not very reactive. Carbon and hydrogen have full valence shells in a C-H bond, meaning it is very stable. Thus,
the bonds are also fairly high energy, causing the compounds to be relatively inert;
➔ Lower alkanes are highly flammable;
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, ➔ They are nonpolar molecules, therefore hydrophobic.
Halogens
Halogens are group 17 elements, consisting of Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At).
Some of the general properties of halogens are listed below:
➔ Electronegativity and reactivity decrease down the group. This is because halogens need to gain one electron in
order to have a full valence orbital. Therefore, more electron shielding and lower effective nuclear charge reduce
the halogens electron affinity;
➔ Atomic radius increases down the group. As the number of orbitals increase, thus the distance from the nucleus to
the valence shell also increases.
Synthesis of halogenoalkanes
There are many methods in which halogenoalkanes can be synthesised, however, the two main reactions include photo-
initiated free radical substitution and electrophilic addition.
Alkanes can undergo photo-initiated free radical substitution to produce halogenoalkanes. Homolytic fission of a
dihalogen compound is initiated by UV light, which causes two halogen free radicals to be produced. These then
propagate more free radical substitution reactions amongst alkane compounds, which may go one indefinitely. The last
stage is the termination stage, which is when two free radicals collide to form a neutrally charged compound.
Below is an example of one of the pathways a reaction may take in the production of bromomethane:
➔ Initiation: the homolytic fission of a bromine molecule under UV light produces two bromine free radicals.
Br2→2Br・
➔ Propagation: the bromine free radical is involved in another free radical substitution reaction, where another free
radical is produced.
Br・+ CH4 → HBr + ・CH3
➔ Termination: the methyl free radical and the bromine radical collide to form bromomethane. Other by-products
may be formed, such as ethane or bromine.
・CH3 + Br・→CH3Br
・CH3 +・CH3 → C2H6
Br・+ Br・→Br2
Alkenes are a much more reactive species due to the high electron density in the double bond. Electrophilic species, such
as hydrogen or halogen atoms, are attracted to this nucleophilic carbon centre, which allows the compounds to undergo
electrophilic addition. This involves the addition of atoms or groups of atoms across the carbon-carbon double bond. The
weaker π bond in the C=C bond is broken, allowing the electrophilic molecule to form bonds with the carbon atoms. An
alkene reacting with a hydrogen halide will produce a monohalogenoalkane. An alkene reacting with a halogen molecule
will produce a dihalogenoalkane. Note that this particular pathway is irrelevant to this investigation, as a C=C bond
cannot be formed within methane.
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