Halogenoalkanes
Halogenoalkanes
Halogenoalkanes are a homologous series of saturated carbon compounds that contain one or more halogen atoms. They are used as refrigerants, solvents, flame retardants, anaesthetics and pharmaceuticals but their use has been restricted in recent years due to their link to pollution and the destruction of the ozone layer.
They contain the functional group C-X where X represents a halogen atom, F,Cl, Br or I. The general formula of the series is CnH2n+1X.
The C-X bond is polar because the halogen atom is more electronegative than the C atom. The electronegativity decreases as you go down group 7 therefore the bond becomes less polar. Flourine has a 4.0 EN whereas iodine has a 2.5 EN meaning it is almost non-polar.
The two types of intermolecular forces between halogenoalkane molecules are Van Der Waals and permanent dipole-dipole interactions. As the carbon chain length increases, the intermolecular forces (due to VDWs) increase as the relative atomic mass increases due to more electrons creating induced dipoles. Therefore the boiling point of the halogenoalkanes increases since more forces must be broken.
Branched chains have lower boiling points than chains of the same length and halogen because the VDWs are working across a greater distance and are therefore weaker.
When the carbon chain length is kept the same, but the halogen atom is changed, despite the effect of the changing polar bond on the permanent dipole-dipole interactions, the changing VDWs have a greater effect on the boiling point. Therefore as RAM increases, the boiling point increases meaning an iodoalkane has a greater boiling point than a bromoalkane if they have the same carbon chain length.
Halogenoalkanes are insoluble or only slightly soluable in water despite their polar nature. They are soluble in organic solvents such as ethanol and can be used as dry cleaning agents because they can mix with other hydrocarbons.
Summary
Halogenoalkanes are saturated carbon compounds with one or more halogen atoms. Their general formula is CnH2n+1X, where X is a halogen. Their functional group is therefore C-X.
They are used as refrigerants, solvents, pharmaceuticals and anaesthetics but have been restricted due to their link to the depletion of the ozone layer.
C-X bonds are polar due to the halogen being more electronegative than the carbon. The polarity of the bond decreases down group 7.
Van der Waals and permanent dipole-dipole interactions are the intermolecular forces in halogenoalkanes.
When carbon chain length increases, boiling points increase due to RAM increasing and the number of Van Der Waals increasing too.
In branched halogenoalkanes, Van Der Waals are working across a greater distance therefore attraction is weaker and boiling points are lower than an identical unbranched chain.
When the halogen is changed, the boiling point increases down the group due to the effect of a greater RAM - more VDWs mean more intermolecular forces to break.
Halogenoalkanes are insoluble in water but soluble in organic solvents like ethanol.
Bonus: free radical substitution reactions in the ozone layer
Ozone, O3, is an allotrope of oxygen that is usually found in the stratosphere above the surface of the Earth. The ozone layer prevents harmful rays of ultraviolet light from reaching the Earth by enhancing the absorption of UV light by nitrogen and oxygen. UV light causes sunburn, cataracts and skin cancer but is also essential in vitamin D production. Scientists have observed a depletion in the ozone layer protecting us and have linked it to photochemical chain reactions by halogen free radicals, sourced from halogenoalkanes which were used a solvents, propellants and refrigerants at the time.
CFCs cause the greatest destruction due to their chlorine free radicals. CFCs – chloroflouroalkanes – were once valued for their lack of toxicity and their non-flammability. This stability means that they do not degrade and instead diffuse into the stratosphere where UV light breaks down the C-Cl bond and produces chlorine free radicals.
RCF2Cl UV light —> RCF2● + Cl●
Chlorine free radicals then react with ozone, decomposing it to form oxygen.
Cl● + O3 —> ClO● + O2
Chlorine radical is then reformed by reacting with more ozone molecules.
ClO● + O3 —-> 2O2 + Cl●
It is estimated that one chlorine free radical can decompose 100 000 molecules of ozone. The overall equation is:
2O3 —-> 3O2
200 countries pledged to phase of the production of ozone depleting agents in Montreal, leading to a search for alternatives. Chemists have developed and synthesised alternative chlorine-free compounds that do not deplete the ozone layer such as hydroflurocarbons (HFCs) like trifluromethane, CHF3.
SUMMARY
Ozone, found in the stratosphere, protects us from harmful UV light which can cause cataracts, skin cancer and sunburn.
Ozone depletion has been linked to the use of halogenoalkanes due to their halogen free radicals.
CFCs were good chemicals to use because they have low toxicity and were non-flammable. The fact they don’t degrade means they diffuse into the stratosphere.
Chlorine free radicals are made when CFCs are broken down by UV light.
These go on to react with ozone to produce oxygen.
Chlorine free radicals are then reformed by reacting with more ozone.
It is a chain reaction that can deplete over 100 000 molecules of ozone.
There is a 200 country ban on their use and scientists have developed alternatives like hydrofluorocarbons to replace them
Happy studying!
More Posts from Amateurchemstudent and Others
finally, some content! this was a quick info graphic I drew up on Procreate to revise for my ochem test tomorrow. disclaimer: I used information from this source (https://www.masterorganicchemistry.com/2010/05/24/imines-and-enamines/) since my own notes are based off lectures I received at my university that I’m not really allowed to share without heavy modification.
general post disclaimer: I’m an undergraduate student studying biochemistry and genetics. Posts are made for the purposes of education, revision and aesthetics. Not all the content I produce can be taken as entirely accurate and I do not take responsibility for errors made as a result of using this resource. Always consult course textbooks and lectures to aid in your specific learning outcomes. Do not repost without the original caption citing any extra references I used to make this post or remove my watermark. Other posts can be found on my blog as-studypeach@tumblr.com. Any problems, feel free to get in touch via my messages.
Nomenclature - what in the organic chemistry is it?
Organic chemistry is so widely studied it requires a standard system for naming compounds, developed by IUPAC. Nomenclature is simply naming these organic compounds.
So, you want to be an organic chemist? Well, it starts here. Are you ready?
(psst… once you’ve learnt this theory, try a quiz here!)
1. Count your longest continuous chain of carbons.
Bear in mind that some chains may be bent. You’re looking for the longest chain of subsequent carbon atoms. This number correlates to root names that indicate the carbon chain length, listed below:
The second part of naming your base comes from the bonding in the chain. Is it purely single bonds or are there double bonds in there? If you are familiar with carbon chemistry, you’ll know that saturated hydrocarbons are called alkanes and unsaturated hydrocarbons are called alkenes. Therefore, the syllable -ane is used when it has only single bonds and the syllable -ene is used when it has some double bonds. For example:
Sometimes carbon chains exist in rings rather than chains. These have the prefix of -cyclo.
2. Identify your side chains attached to this main carbon and name them.
Side chains are added as prefixes to the root names. Sometimes called substituents, these are basically anything that comes off the carbon chain. Examples of the prefixes are listed below:
There are other prefixes such as fluoro (-F) and chloro (-Cl) which can describe what is coming off the chain.
3. Identify where each side chain is attached and indicate the position by adding a number to the name.
We aim to have numbers as small as possible. For example, if bromine is on the second carbon of a 5-carbon saturated chain, we number it as 2-bromopentane instead of 4-bromopentane, since it would essentially be 2-bromopentane if it was flipped. Locant is the term used for the number which describes the position of the substitute group, e.g. the ‘2′ in 2-chlorobutane is the locant.
Sometimes there are two or more side chains e.g. a methyl group and a chlorine attached to a pentane. In these cases, these rules apply:
1. Names are written alphabetically.
2. A separate number is needed for each side chain or group.
3. Hyphens are used to separate numbers and letters.
This would be named 2-chloro-3-methyl-pentane. This is because the longest chain of carbons is 5 (pentane), the chlorine is on the second carbon (2-chloro) and the methyl group is on the third carbon (3-methyl). It is 2-chloro rather than 4-chloro as we aim to have as small as numbers as possible.
Another variation of this step to be aware of is how many of the same side chains or groups there are, for example, having two methyl groups would be dimethyl rather than solely methyl. Each group must also be given numbers separated by commas to show where each one is located.
The list of these prefixes is found here:
Convention does not usually require mono- to go before a single group or side chain.
4. Number the positions of double bonds if applicable.
Alkenes and other compounds have double bonds. These must be indicated with numbers. For example, pent-2-ene shows that the double bond is between carbon 2 and carbon 3. The number goes in the middle of the original root name e.g. butene, pentene.
(!) Below is a list of functional groups that you may need to study for the AS and A Level chemistry exams. “R” represents misc. carbons. It is important to know that some groups are more prioritised than naming. From the most to least priority: carboxylic acid, ester, acyl chloride, nitrile, aldehyde, ketone, alcohol, amine, alkene, halogenalkane. It is worthwhile learning these.
bigger version here (I suggest downloading and printing it)
But wait, there’s more:
Here are some things to bear in mind when naming organic compounds:
1. The letter ‘e’ is removed when there are two vowels together e.g. propanone rather than propaneone. The ‘e’ isn’t removed when it is next to consonant, e.g. propanenitrile isn’t propannitrile.
2. When compounds contain two different, one is named as part of the unbranched chain and the other is named as a substituent. Which way round this goes depends on the priority.
SUMMARY
Count your longest continuous chain of carbons.
Chains may be bent. You’re looking for the longest chain of subsequent carbon atoms. This number correlates to root names that indicate the carbon chain length, e.g. pentane.
The second part of naming your base comes from the bonding in the chain. Is it purely single bonds or are there double bonds in there? The syllable -ane is used when it has only single bonds and the syllable -ene is used when it has some double bonds.
Rings have the prefix of -cyclo.
Identify your side chains attached to this main carbon and name them.
Side chains are added as prefixes to the root names. Sometimes called substituents, these are basically anything that comes off the carbon chain.
There are other prefixes such as fluoro (-F) and chloro (-Cl) which can describe what is coming off the chain.
Identify where each side chain is attached and indicate the position by adding a number to the name.
We aim to have numbers as small as possible. Locant is the term used for the number which describes the position of the substitute group, e.g. the ‘2′ in 2-chlorobutane is the locant.
Sometimes there are two or more side chains e.g. a methyl group and a chlorine attached to a pentane. In these cases, names are written alphabetically, a separate number is needed for each side chain or group and hyphens are used to separate numbers and letters.
When there are two or more of the same side chains or substituent groups, these must also be given numbers separated by commas to show where each one is located.
Number the positions of double bonds if applicable.
Alkenes and other compounds have double bonds. These must be indicated with numbers. The number goes in the middle of the original root name e.g. butene, pentene.
It is worthwhile learning the other functional groups that can be added on.They have varying priorities.
The letter ‘e’ is removed when there are two vowels together e.g. propanone rather than propaneone. The ‘e’ isn’t removed when it is next to consonant, e.g. propanenitrile isn’t propannitrile.
When compounds contain two different, one is named as part of the unbranched chain and the other is named as a substituent. Which way round this goes depends on the priority.
Happy studying guys!
Follow @productive-tips for more tips and content like this posted daily! Handpicked and curated with love :)
Slice of Life
#Thunderstorms are hitting the UK this week – here’s how thunder and lightning happen and some of the chemistry going on during a storm: https://ift.tt/2XUCKZc https://ift.tt/3gJ7ALD
The two kinds of water
Credit: University of Basel
Pre-sorted ortho-water and para-water molecules with differently oriented nuclear spins (blue or red arrows) react with diazenylium ions (centre left) at different speeds.
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Researchers from the University of Basel’s Department of Chemistry, Switzerland, has investigated how the two forms of water differ in terms of their chemical reactivity – the ability to undergo a chemical reaction. Both forms have almost identical physical properties, which makes their separation particularly challenging.
It is less well-known that water exists in two different forms (isomers) at the molecular level. The difference is in the relative orientation of the nuclear spins of the two hydrogen atoms. Depending on whether the spins are aligned in the same or opposite direction, one refers to ortho- or para-water.
The was made possible by a method based on electric fields. Using this, researchers were able to initiate controlled reactions between the pre-sorted water isomers and ultracold diazenylium ions (protonated nitrogen) held in a trap. During this process, a diazenylium ion transfers its proton to a water molecule. This reaction is also observed in the chemistry of interstellar space.
It was discovered that para-water reacts about 25% faster than ortho-water. This can be explained in terms of the nuclear spin also influencing the rotation of the water molecules. As a result, different attractive forces act between the reaction partners. Para-water is able to attract its reaction partner more strongly than the ortho-form, which leads to an increased chemical reactivity.
i just learned from animal crossing that pondskaters stay on top of the water by secreting an oil from their feet
that seems kinda obvious in hindsight. i always figured they were just, like, light enough to not break surface tension
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