Block 7 Alkenes and aromatics

Pregunta	Respuesta
What key character do alkenes have?	They have one or more C=C double bond.
What key character do alkynes have?	They have one or more CC triple bond.
What kinds of bonds are formed in a double bond?	A sigma bond and a Pi bond.
How is a sigma bond formed?	By the overlapping of two sp2 hybridised orbitals.
How is a Pi bond formed	By the overlapping of the remaining unhybridised P orbitals present.
Which is stronger, Sigma or Pi?	The sigma bond is the strongest.
What are allenes?	Allenes are alkenes that contain two adjacent CC double bonds and are not planar.
Are compounds with even numbers of adjacent double bonds planar or non-planar?	They are non-planar. Coimpounds with odd numbers including one are planar.
Why is a simple alkyne a linear compound and what is the bond angle of CH?	There is only one H or group attached to the remaining bond of the C atoms. This group moves as far away as possible which is at an angle of 180 degrees.
What are stereoisomers?	Two compounds that have the same chemical formula but have a different arrangement of atoms in space.
Why do asymetrical substuted alkenes display isomerism?	Rotation around the double bond is energetically unfavourable as the weaker Pi bond would have to break and reform.
But-2-ene forms two diastereomers. What is a diastereomer and what two forms do they come in??	A diastereomer is a geometric stereomer. But-2-ene comes in E and Z forms.
The electrons of which bonds dominate the chemistry of alkenes and alkynes?	The electrons of the Pi bonds.
The Pi bond of the alkene can be thought of a an electron dense, negatively charged, cloud above and below the carbon framework. What does this provide in simple alkenes?	An electron rich centre that repels nucleophiles and attracts electrophiles.
Why is the electron cloud of the Pi orbital important in alkene chemistry?	Alkenes tend to react by mechanisms involving nucleophiles and electrophiles.
Define nucleophile.	A species with at least one non-bonded pair of electrons which ultimately reacts to form a new bond with carbon.
Define electrophile	A fully or partially charged species that reacts with a nucleophile.
What are addition reactions?	Reactions where atoms or groups add to a molecule containing a double or triple bond.
What do addition reactions do?	They reduce the amount of unsaturation in a compound.
Name a characteristic addition reaction of alkenes?	electrophilic addition where many reagents can add across a double bond such as halogens and hydrogen halides.
When a reagent X2 is added across the double bond of an alkene, which is the nucleophile and which is the electrophile?	The alkene is the nucleophile and the reagent is the electrophile in this case. ( reactions are generally named after the nature of the reagent.
Why is the reagent the electrophile in the previous example?	Even though the Pi electrons are bonding electrons they do react with electrophiles because they are far enough away from the carbon nuclei to be susceptible to electrophiles. They are polarisable and easily influenced by charged species in the vicinity.
Is the mechaism by which electrophile additions proceed one step or two?	It is two step. Step 1. often reaction between alkene and electrophile. Step 2. Attack of the nucleophile on the carbon intermediate.
What is a carbocation?	A species bearing a carbon atom with a positive charge.
How many intermediates/products can the addition of a simple asymetrical alkene lead to?	2 as the proton can bond either to the central carbon atom on one side of the double bond or the terminal carbon atom on the other side. This leads to two possible intermediates and therefore two possible products. (see Fig 2.1)
In practice one product is produced more than the other. Why is this?	It depends on the relative stability of the two possible intermediate carbocations.
What is Markovnikov' rule?	When adding HX to an unsymetrical alkene across a double bond the hydrogen always adds to the less substituted carbon of the double bond.
What is the saying to help you remmber Markovnikov's rule?	'To the one who has will more be given.'
Markovnikov's rule applies specifically to the addition of HX to an unsaturated system. How can the major product of any electrophilic addition reaction be predicted?	By looking at carbocation stability.
In electrophilic addition reactions does the major product arise from the most stable or least stable carbocation?	From the most stable carbocation.
What is the order of stability for carbocations?	Tertiary>secondary> primary> methyl. with tertiary as the most stable and methyl as the least.
Why are tertiary carbocations the most stable?	Alkyl groups are electron donating. The inductive donating effect of three alkyl groups attached to the central positively charged carbon atom helps to stablize it. There is a larger transfer of electron density.
What role do carbocations play in electrophilic addition reactions and what do they react with?	Carbocations are reactive intermediate species that react with nucleophiles to form the final product.
Why are more stable carbocations more likely to form the major product?	The activation energy is lower for a more stable carbocation intermediate. (Based on the assumption that the energy of the transition state is the same as the energy of the carbocation intermediate.)
The second step from intermediate to product is a fast step. What does this say about the activation energy?	It has a low activation energy when compared to the first step.
The first step is reversible due to the high activation energy. What does the low activation energy mean for the second step?	It is not reversible. The energy of activation of the reverse reaction is so large that conversion of the product back to the carbocation is too slow to be measurable.
In electrophilic addition reactions why does the major product form from the most stable carbocation intermediate?	That reaction pathway has the lower initial activation energy. The reactions are controlled kinetically and therefore the major product is the one that is formed faster.
What are resonance structures?	Structural formulae that represent the average of 2 or more structural formulae that cannot be represented by a single formulae due to bond lengths. Or molecules that are represented by structures that differ only in the position of the Pi electrons while the framework and overall charge remain the same.
What is resonance a property of?	Resonance is a property of systems containing Pi electrons.
What do resonance structures show?	They are representations rather than separate compounds. They are a convenient way of showing that the actual molecular structure is a composite and an average of all the possible forms that could be drawn. Also that the bond distances are equal and intermediate between single and double bonds.
How can resonance structures be interconverted?	By drawing curly arrows. (e.g. two benzene rings with alternating double bonds - each ring having the double bonds in the alternate positions- can have curly arrows showing the movement of the electrons from each double bond to the adjacent single bond.)
For Pi systems, what do resonance structures demonstrate?	That electron density can be spread over a larger area or delocalised.
What does delocalisation do for the molecule?	It increases the stability of the molecule. The larger the area over which the electrons are spread, the more stable the molecule.
How does resonance and the delocalisation of electron density affect a carbocation?	Carbocations are also stablised by resonance. The charge can be delocalised over a group of atoms or part of a structure.
Why does this delocalisation increase stability of carbcations?	It lowers the overall energy of the molecule so molecules that experience resonance are more stable than those that don't.
Curly arrows are used differently when drawing resonance structures to when they are used for drawing reaction mechanisms. How is this so?	When curly arrows are used for reaction mechanisms they represent the movement of electron pairs. For resonance structures there is no real movement of electrons, they are a convenient way of drawing different structures. THe real molecule is an average of all the resonance structures that can be drawn.
What can act to stablize or destableize a carbocation?	By inductive donation of the alkyl groups or substituton (tertiary most stable). Resonance stablization. Electron withdrawal e.g. from a carbonyl group. (See exercise 2.4)
Name an electrophilic addition reaction that results in the formation of alcohols?	The hydration of alkenes, adding water across the double bonds results in alcohols and is also an electrophilic addition reaction.
In what substance are industrial hydration of alkenes reactions usually carried out?	Dilute aqueous sulfuric acid.
What is the first step of the hydration of the alkene reaction?	The protonation of the alkene. (With H+)
The hydration of an alkene is also a electrophilic substitution reaction. What part of the carbocation acts as the electrophile?	The proton.
IN the same hydration reaction, what acts as the nucleophile and what does it attack? what is lost by the carbocation to form the final product?	The water molecule attacks the carbocation and the final product is formed when the carbocation loses the proton.
What are hydration reactions the reverse of?	Acid catalysed elimination reactions of water from alcohols. The reaction can be thought of as an equilibrium.
What conditions can effect the equilibrium of the hydration/elimination reaction?	Low temps and a reaction in a dilute aqueous solution favours alcohol formation. Elimination is favoured at high temps and can allow for evaporation of the alkene from the reaction mixture.
Why does reaction in an aqueous solution favour the formation of the alcohol?	Because the nucleophile is water and it is more likely to react with the carbocation if water is in higher concentrations.
How can this reaction be used to form a more substituted alkene from a less substituted one?	The balance between elimination and addition can be used to protonate a double bond on one part of the alkene and deprotonate another double bond elsewhere on the molecule. The most thermodynamically stable (usually most substituted alkene) alkene is favoured.
In addition of halogen reactions which is the electrophile and which is the nucleophile?	The halogen provides the electrophile and the alkene provides the nucleophile.
How does the halogen provide the nucleophile?	Halogens are non polar covalent molecules but when they get close to a double bond they become polarised. Electrons form the double bond go to the Br+ in the first stage of the reaction. The remaining polarised Br- then attacks the resulting carbocation (in role of nucleophile.
Why might you get a lack of stereoselectivity in an addition reaction of Z but-2-ene?	The carbocation formed in the first step might be attacked from below or above. Of the 3 possible diastereomers, the two enantiomers (which are equivilent) and the meso form would be formed resulting in two products.

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