33

Dry ice (solid carbon dioxide) sublimes at −78 °C. Dry ice and acetone are a common cold bath for chemical reactions. The melting point of acetone is -95 °C so the bath never gets cold enough to freeze the acetone. The bubbling of the carbon dioxide gas as the dry ice sublimes keeps the cold bath well stirred.


31

Thermodynamic and kinetic control This is a classic example of the concept of thermodynamic versus kinetic control of a reaction. Take a look at this energy profile diagram.1 The horizontal axis is a reaction coordinate, and the vertical axis represents Gibbs free energy. In this scenario, the starting material $\ce{A}$ can react to form either $\ce{B}$ or ...


22

An $\alpha,\beta$-unsaturated ketone is electrodeficient at the $\beta$ position. This can be seen if you draw the resonance structures of such a molecule. The $\beta$ carbon is thus a good site for nucleophilic attack. But, as you know, carbonyls are also prone to nucleophilic attack. To discriminate between the two, you need to look at how the reaction is ...


12

Curt F. is probably correct in his estimate that heptafluoropropane extinguishes fire primarily by physical means. According to an overview by Choy and Fong, 'An Introduction to Clean Agents: Heptafluoropropane' (Int. J. on Eng. Performance-Based Fire Codes, vol. 5, nr 4, p. 181$-$184, $2003$), For heptafluoropropane, the contribution of physical ...


12

This is not really my area of expertise, but a quick search for "python chemical reactions" revealed three hits I've never seen before that may be of interest, the first one being closest to what you want. chempy from chempy import ReactionSystem # The rate constants below are arbitrary rsys = ReactionSystem.from_string("""2 Fe+2 + H2O2 -> 2 Fe+3 + 2 ...


12

The colour of the filtrate after filtering out the solid is still lightly blue in colour. This indicates that Copper (II) sulfate still is present in the solution, meaning that there was not enough iron to completely react with all the copper (II) sulfate, visually confirming it to be the limiting reagent. A solution containing only Iron (II) sulfate is ...


11

Diesel engines vs. gasoline engines Diesel engines don't rely on spark plugs, but they still work by igniting the fuel to generate a force that moves a cylinder in the engine. The air/fuel mixture in a diesel engine is ignited by the compression of the same cylinder during the "compression" stroke. That is, like most gasoline engines in cars, diesel ...


10

It must first be understood that Michael Addition is a special type of conjugate (1,4) addition where the nucleophilic species is an enolate. This will undergo 1,4 addition to the A-B unsaturated carbonyl. Organocuprates add in a 1,4 fashion. The mechanism behind this is not fully understood, but it is known that radical intermediates are involved. ...


10

Your method and your mathematics seem perfectly fine, and your calculated result is correct. You are also quite correct that the result will change depending on the value of $E_\mathrm a$ that you choose. The problem you're running into is the assumption that the stated rule of thumb holds exactly, regardless of temperature. It doesn't. I would set up the ...


10

I suspect $\ce{HBr}$ is produced in situ from $\ce{KBr}$ and $\ce{H2SO4}$. Same process isn't particularly suitable for $\ce{HI}$ generation as it reacts with $\ce{H2SO4}$: $$\ce{2HI + H2SO4 -> H2SO3 + H2O + I2}$$ I actually seriously doubt second reaction will take place at all. Hydroiodic acid can also act as a reducing agent, converting the ...


8

The nature of the substrate (in particular the degree of substitution of the α-carbon) is of principal importance in determining which mechanism predominates. Generally, $\mathrm{S_N1}$ and $\mathrm{E1}$ are both less common than $\mathrm{S_N2}$ and $\mathrm{E2}$, partially as a consequence of their first-order kinetics and the stability constraints imposed ...


8

There are always two sides of a reaction. In the case of an alcohol attacking a sulphonic acid chloride, we can look closely at the alcohol and say: ‘Oh my, this is a bad nucleophile and we cannot deprotonate it with the conditions we have. Probably, this reaction won’t work’. But we can also look at the sulphonic acid chloride and say: ‘Oh my, this is a ...


8

Before considering a double pendulum, let us consider a simple pendulum, which is modeled by $$\ddot \theta + \frac{g}{\ell} \sin (\theta) = 0$$ Choosing units that make $\frac{g}{\ell} =1$, we have $$\ddot \theta + \sin (\theta) = 0$$ Let $x_1 := \theta$ and $x_2 := \dot \theta$. Hence, the 2nd order ODE above can be rewritten as follows $$\begin{array}...


8

It sort of depends what you mean. If by "affect the rate of a chemical reaction", you mean affect the individual steps of some overall reaction as to slow them down, then no, gravity definitely has no effect at all. This is simply because the gravitational force is so much weaker than the electrostatic force, which will dominate all interactions relevant in ...


7

That wikipedia page on gaseous fire suppression is not very good. It is very difficult for me to believe that pentafluoroethane has a different mechanism of fire suppression than heptafluoropropane. I suspect that all the inert-gas agents work by lowering oxygen concentration, both by simple dilution and by virtue of their density selectively displacing ...


7

The temperature at which many reactions are carried out is often arbitrary, with certain values being favoured due to the ease at which cooling baths can be prepared. As an example -78°C is highly prevalent, and a question about this 'magic temperature' has been asked here previously. In the case of reactions forming diazonium salts, there has been a ...


6

Background First off, let's discuss what we mean by "kinetic" and "thermodynamic" products. The kinetic product is the first formed product, the one that forms the fastest, the one with the largest rate constant. The rate constant, k, can be described using the Arrhenius equation $$\mathrm{k=Ae^{\frac{-Ea}{RT}}}$$ where A is the pre-exponential factor, a ...


6

The product on the far left cannot form in any reasonable amounts. Consider the HOMO of s-cis-butadiene (which is a close enough model system to your dimethylcyclohexadiene): Those contributions that extend into space furthest are those on the terminal carbons. (Image credit: ChemTube3D.com by Nick Greeves. Ignore the right half of the image, it is not ...


6

Diesel fuel The difference between gasoline and diesel fuel lies in the length of carbon chains. Gasoline and diesel fuel are examples of hydrocarbons, molecules made up purely of hydrogen ($\ce{H}$) and carbon ($\ce{C}$) atoms. Simple examples include $\ce{C_3H_8}$ (propane) and $\ce{C_8H_18}$ (octane). Gasoline contains compounds with carbon chains ...


6

Any galvanic cell produces its equilibrium voltage at open-circuit. Let's say your cell is at +3 V before you allow any current to flow through it. As soon as you throw the switch, you allow that 3 V to push a current through the cell and some external load that you connect to it. There are several resistances within the cell that act to reduce the cell ...


6

There is a book The Design of Controlled-atmosphere Chambers for the Study of Oxygen Toxicity, so I suggest "contolled-atmosphere chamber".


5

In aqueous medium, phenol is deprotonated to a certain extent, forming the phenoxide ion in which the ortho and para positions are even more activated than in phenol itself. Hence, trisubstitution occurs here. $$\ce{PhOH + H2O <<=> PhO- + H3O+}$$ In an organic solvent, phenol is not deprotonated and monosubstitution tends to occur. ron also ...


5

ChemGuide has a good introductory article here. The effects of increasing pressure and temperature are, to an extent, equivalent. Increased pressure leads to increased collisions and increased collision strength between molecules, allowing the (usually high) activation energy barrier to be overcome at a noticeable rate; at standard temperatures and ...


5

Comparison of the two approaches as far as yield/cost/etc. is void because one of them does not provide the same product as the other. Hydroboration-oxidation of 1,4-dimethylenecyclohexane yields 1,4-bis(hydroxymethyl)cyclohexane with undergoes oxidation to yield 1,4-cyclohexanedicarbaldehyde or 1,4-cyclohexanedicarboxylic acid. Now, you may be able to take ...


5

There's no way to predict this. Mnemonics are just rules, and rules are just rules. There will be exceptions, and you won't know when you should apply them a priori. If you could, it would be part of the rule by definition. The Curtin-Hammett conditions apply when the rate of interconversion between intermediates is significantly faster than the rate of ...


5

First of all, this question is very interesting. Though the question looks very simple but it is very complicated if looked upon carefully. I am just going to shed light on some points in addition to @andselisk's and @Jan's excellent answers. @andselisk said that $\ce{HBr}$ is produced in situ from $\ce{KBr}$ and $\ce{H2SO4}$. So, why not write in this ...


5

The synthesis of alkyl halides from the corresponding aliphatic alcohols using concentrated hydrohalogen acids was investigated by Klein, Zhang and Jiang.[1] They note: [W]e found that the reflux of 1-butanol ($\pu{2.34 g}, \pu{31.5 mmol}$) with $48~\%$ hydrogen bromide ($\pu{7 mL}$) for $\pu{4 h}$ on a $\pu{120 °C}$ oil bath only gave low yield of 1-...


4

Yes. Pressure, as a concept, is simply the force exerted by random molecular motion per unit area, and since all matter is composed of molecules, and all matter above absolute zero has some random motion, everything has molecules exerting some force over some area. (This pressure is the vapor pressure of the solid, which is typically extremely low. If the ...


4

You should look in some kind of reaction rate database, e.g. NIST. An example from the database:


4

Probably both mechanisms can happen and it would require experimental data from reaction kinetics and computational calculations to determine which one of the mechanism is domintant. One example of determined mechanism about neighboring group effect, also called anchimeric assistance is mustard gas (chloromethine) alkylation reaction with DNA. ...


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