What Are Volatile Organic Compounds
Volatile organic compounds are compounds that have a high vapor pressure and low water solubility. Many VOCs are human-made chemicals that are used and produced in the manufacture of paints, pharmaceuticals, and refrigerants. VOCs typically are industrial solvents, such as trichloroethylene fuel oxygenates, such as methyl tert-butyl ether or by-products produced by chlorination in water treatment, such as chloroform. VOCs are often components of petroleum fuels, hydraulic fluids, paint thinners, and dry cleaning agents. VOCs are common ground-water contaminants.
Volatile organic compounds are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors than outdoors. VOCs are emitted by a wide array of products numbering in the thousands. Examples include: paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.
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I don’t know about Kevin but here’s the correct answer: It represents a substituent group derived from an alkane. For example if you had a simple propane chain with a CH3 substituent on the #2 carbon you would have 2-methylpropane, similarly if you had a benzene ring attached to a C which itself has a ketone and Cl group on it, it would be called “benzoyl chloride” due to the benzene attached to the C.
I know what kevin means by “radical” but in the context provided, it is the substituent itself i.e. the methyl and benzoyl groups of before. I know what kevin is saying and it’s correct to an extent but let me elaborate. A “radical” by its correct definition is any chemical species that contains one or more unpaired electrons due to bond cleavage, and while a carbon radical will have a “yl” suffix other radicals exist which don’t such as Cl whose radical is named monochlorine.
If you are studying nomenclature, you are pretty new to organic so don’t worry about radicals yet, you’ll get to them soon enough.
Take my advice to the test, and while I know Kevin has good intentions, there is nothing that can be gained by regurgitating information googled online for no particular reason.
Hope this helped.
Double And Triple Bonds In The R And S Configurations
Lets do the R and S for this molecule:
Bromine is the priority and the hydrogen is number four. Carbon a is connected to one oxygen and two hydrogens. Carbon b is connected to one oxygen and one hydrogen. However, because of the double bond, carbon b is treated as if it is connected to two oxygens. The same rule is applied for any other double or triple bond. So, when you see a double bond count it as two single bonds when you see a triple bond cut it as three single bonds.
The arrow goes clockwise, however, the absolute configuration is S, because the hydrogen is pointing towards us.
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Why Do We Bother With This Ancient Nomenclature
You might justifiably ask: dont we already have a system for assigning absolute configuration ? Why do we need a new system?
The D-L system isnt a new system, folks. Its the old system it predates Cahn-Ingold-Prelog.
The D-L system is literally a remnant of the horse-and-buggy era, dating back to Emil Fischers work on carbohydrates in the late 1800s a time when organic chemists had no way to determine the absolute configuration of stereocenters, which only became possible in 1951 .
So why does it still get used? Shouldnt it be consigned to the dustbin of history, along with slide rules, 8-track cassettes, and 5 ¼ floppy disks?
Well, there are thriving communities in parts of rural America where horse-drawn carriages persist if you know where to look.
Likewise there is a pocket of organic chemistry where D-L system still finds use, and that is specifically in the realm of sugars and amino acids.
This not a revolt by Amish chemists against the modern evils of the CIP system, by the way. There are at least 3 good reasons, in the specific case of sugars and amino acids, for using L- and D- :
It bears repeating: with sugars and amino acids, L- and D- can be useful designations. For other molecules, you can largely forget about it.
Important Concepts In Organic Chemistry
Essential oil chemistry is a part of organic chemistry, which covers a vast range of compounds. Early ideas suggested that organic compounds were all obtained from either plant or animal sources, i.e. that they were natural products, and arose only through vital forces inherent in living cells. This definition is no longer true as a result of modern laboratory synthetic methods. The modern definition of organic chemistry is that it is the chemistry of covalently bonded carbon compounds.
There is a very dangerous misconception that if a substance is natural it is not harmful, and can be used therapeutically without the fear of any side-effects. Natural products often contain very powerful and toxic compounds and some form a basis for mainstream drugs. Essential oils are very complex mixtures of organic compounds, many of which should be used with great care. Their roles in the plant body are often protective and defensive, e.g. to repel invading organisms. Essential oils contain compounds with varying physiological effects and toxicity. In a genuine aromatherapy-grade essential oil, the more toxic components are often balanced by others that act as quenchers. There is a phenomenon called synergy whereby the components making up the oil can cooperate to produce their healing effect. A knowledge of the oil components is needed for their safe use and this is why it is vital to use high-grade essential oils in a controlled way
2The bonding is mainly covalent
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What Is Reagent Grading
You may already know that not all chemicals used in the lab are pure. Therefore, the grading system is used to show how pure a substance is. High grades are provided to the purest chemicals. As the level of impurities increases, the grades begin to get low. These impurities can be metal, water, or other chemicals.
Validated methods specify the grade of reagents to be used. It is important to use specified grades otherwise, errors can arise due to contamination from reagents themselves. On the other hand, you can incur additional costs in the analysis if you use a superior grade of reagent when your analysis does not have such high purity requirements.
Energetic Measure Of Aromaticity
Already in the nineteenth century, it was known that aromatic compounds are much more resistant to chemical reactions than their acyclic analogues . First quantitative description of aromaticity was proposed in 1933 by introduction of a thermodynamic term, namely the resonance energy, RE i.e. the energy by which the aromatic compound is more stable than its virtual olefinic analogue. In the case of benzene, this analogue is a virtual compound with three single and three double bonds. Estimated RE for benzene amounts to 36 kcal/mol. A very similar value was experimentally determined by Kistiakowsky et al. through calorimetric measurements of heats of hydrogenation of benzene and cyclohexene .
Later, the term RE was replaced by more precisely defined aromatic stabilization energy which is estimated by the use of either isodesmic or more precise homodesmotic reactions. The latter is defined as a virtual reaction leading to products with the same number of CH bonds and the same numbers of atoms in the appropriate hybridization states .
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Six Carbon Aldehyde D
If there are 4 aldopentoses, each as a D- L- pair of enantiomers then how many aldohexoses are there?
Here are the D-aldohexoses. Note how they all have the same configuration of the bottom chiral centre the same one we saw in D-glyceraldehyde.
In contrast to the D-sugars, the L- sugars are rarely found in nature. Interestingly, L-glucose has been explored as a sugar substitute. Its taste is indistinguishable from naturally occurring D-glucose, but provides no nourishment since it cannot be broken down by our enzymes. As it turns out production is just too expensive to compete with splenda, stevia, aspartame et. al.
OK, thats enough. Seven-carbon sugars have been made but theyre not biologically significant.
Four Carbon Aldehyde D
Once the absolute configurations of L- and D- glyceraldehyde were proposed, the absolute configurations of other chiral compounds could then be established by analogy .
Its not crucial for today, but for an example of this kind of reasoning, see this .
Back when the concept of chiral centers was being introduced, you likely learned that a molecule with n chiral centers will have 2n stereoisomers .
Here they are. The important thing to note in the figure below is that the L-family of sugars has the OH group of the bottom chiral carbon on the left, and the D-family has the OH group of the bottom chiral carbon on the right .
See how L-Erythrose and L-Threose build on the stereocenter established in L-glyceraldehyde , and D-Erythrose and D-Threose build on the stereocenter established in D-glyceraldehyde .
Sugars are built up a little like the binary system you can think of each stereocenter is a bit that can have one of two values. The configuration of L-erythrose and L-threose only differs at one stereocenter. If we were to flip the position of H and OH, wed get the other. This relationship has a name that you might see sometimes: two molecules that have the opposite configuration at just one stereocenter are called epimers, particularly when one of the atoms attached to the stereocenter is a hydrogen .
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R And S When The Lowest Priority Is A Wedge
You have two options here:
Option one. Turn the molecule 180o such that the hydroxyl is now pointing towards you and the hydrogen is pointing away. This allows to have the molecule drawn as needed the lowest priority pointing backward as it is supposed to be for determining the R and S configuration:
Next, assign the priorities chlorine-number one, oxygen-two, carbon-three and the H as number four.
The arrow goes clockwise, therefore the absolute configuration is R.
The problem with this approach is that sometimes you will work with larger molecules and it is impractical to redraw the entire molecule and swap every single chirality center.
For example, look at biotin with all these hydrogens pointing forward. Not the best option to redraw this molecule changing all the hydrogens and keeping the rest of the molecule as it should be.
This is why we have the second approach which is what everyone normally follows.
Here, you leave the molecule as it is with the hydrogen pointing towards you. Continue as you would normally do by assigning the priorities and drawing the arrow.
The only thing you have to do at the end is change the result from R to S or from S to R.
In this case, the arrow goes counterclockwise but because the hydrogen is pointing towards us, we change the result from S to R.
Of course, either approach should give the same result as this is the same molecule drawn differently.
Five Carbon Aldehyde D
The most familiar name on that list should be ribose, which is the sugar backbone of ribonucleic acid .
On the left hand side in the diagram below, we have the L-aldopentoses, which all share the same configuration of the bottom stereocenter when the aldehyde is placed at the top.
Their enantiomers, the D-aldopentoses, are on the right hand side, which all share the same configuration of the bottom stereocenter .
At this point we should point out that the overwhelming majority of sugars in Earth-based life forms are D- sugars, including D-ribose as the backbone of RNA. Why and how all organisms on earth ended up with D-sugars is a mystery, as one presumes that L-sugars would have worked just as well.
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Read A Brief Summary Of This Topic
argon , chemical element, inert gas of Group 18 of the periodic table, terrestrially the most abundant and industrially the most frequently used of the noble gases. Colourless, odourless, and tasteless, argon gas was isolated from air by the British scientists Lord Rayleigh and Sir William Ramsay. Henry Cavendish, while investigating atmospheric nitrogen , had concluded in 1785 that not more than 1/120 part of the nitrogen might be some inert constituent. His work was forgotten until Lord Rayleigh, more than a century later, found that nitrogen prepared by removing oxygen from air is always about 0.5 percent more dense than nitrogen derived from chemical sources such as ammonia. The heavier gas remaining after both oxygen and nitrogen had been removed from air was the first of the noble gases to be discovered on Earth and was named after the Greek word argos, lazy, because of its chemical inertness.
Molecular: An Augmented Reality App For Organic Chemistry
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I made an app! MoleculAR is an Augmented Reality app for iOS and Android that gives you the ability to visualize and interact with molecules. Point your devices camera at a MoleculAR structure in your lecture notes, and an interactive, 3D representation of that molecule will appear before your eyes!
The molecule can be manipulated the following ways:
Dont forget to move around and look at your molecules from different angles!
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The Other Indices Of Aromaticity
In addition to the above-presented characteristics of aromaticity it is necessary to briefly mention some other measures, which do not their origin in the enumerative definition presented in the beginning of this paper. The Bader quantum theory of atoms in molecules allows to analyze charge distribution in molecules. Among many properties accessible by the use of this method, the most useful for structural studies are the charges in the atomic basins and properties in the critical point of bonds and rings. The critical points are characterized by the local extreme of electron density, being a minimum charge density in direction of the bond and maximum in directions perpendicular to the bond . This is the so-called the bond critical point , moreover, the ring critical point can also be determined . In addition, it is also possible to compute the density of electron energy in the critical point. It was shown that for polycyclic benzenoid hydrocarbons the QTAIM parameters in RCP i.e. charge, total, kinetic and potential energies very well correlate with HOMA and with NICS with cc = 0.909 .
QTAIM also allows to describe ellipticity of a bond in its BCP. It is known that the more double is the bond, the higher is its ellipticity. Thus the next aromaticity parameter based on elipticity, EL was proposed which successfully correlated with other aromaticity indices like HOMA, EN, GEO, PDI FLU and NICSs. Similar approach was earlier presented , although not so well documented.
What Is The R And S Configuration And Why Do We Need It
However, they dont look exactly the same as the Cl atom points in different directions wedge and dash. These molecules are not the same compound they are non-superimposable mirror images which are known as enantiomers:
The problem with the wedge and dash notation is that it is not a universal approach and quickly loses validity when we simply look at the molecule from the opposite direction:
So, we need an extra piece of information to distinguish enantiomers by their names properly addressing the stereochemistry as well.
Cahn, Ingold, and Prelog developed a system that, regardless of the direction we are looking at the molecule, will always give the same name .
And that is why this is also known as the absolute Configuration or most commonly referred to as the R and Ssystem.
Lets see how it works by looking first at the following molecule and we will get back to the 2-chlorobutane after that:
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Organic Chemistry Of Graphene
Organic chemistry of pristine graphene is an emerging area where functionalization of the graphene surface with small molecules is used to tune the optoelectronic properties of graphene. Although Hummers method uses the oxidation of graphene layers, it produces a severely oxygenated surface, and thus, the electronic and mechanical properties of the resulting GO are significantly different from those of graphene. The rich chemistry of GO functional groups is efficiently explored for functional materials in diverse fields including chemistry, physics, and biology, as well as materials science.42 The poor dispersive nature of unoxidized graphene in solvents was a challenge until successful liquid-phase exfoliation techniques were introduced. The reactions of CVD-grown graphene and exfoliated graphene43 with small molecules are still being explored, as discussed below.
Figure 6.4. Some organic reactions of pristine graphene.
Table 6.4. List of Organic Reactions of Pristine Graphene and Characterization Techniques