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Orgo ch 4
| Question | Answer |
|---|---|
| What happens to the melting point, boiling point, and density as the molecular weight of a compound increases? | They all increase. |
| Describe how long straight-chain compounds you would find as a gas, liquid, and wax/solid at room temp. | 1-4 are gaseous, 5-16 carbons are liquid, longer are waxes/solids. |
| What affect do branched molecules having on boiling points? | They have a slightly lower boiling point than their straight chain isomers.This is because greater branching reduces the surface area of the molecule available for interactions with neighboring molecules. The weakened IM forces result in lower bp. |
| What is the affect of branches on mp? | It tends to also have lower mps than straight chains. However, the more symmetrical a molecule, the higher the mp. |
| What is combustion? | The reaction of alkanes with oxygen to form carbon dioxide, water, and heat. |
| Describe the mechanism of combustion? | It is too complex for the MCAT, but it is believed to proceed through a radical process. |
| What is the equation for combustion? | C3H8 + 5O2 → 3CO2 + 4H2O + heat |
| What happens when combustion is incomplete, which it often is? | It produces carbon monoxide, polluting the air. |
| What are the steps of free radical halogenation of alkanes? | Initiation, Propagation (which can occur many times), and Termination |
| What happens during Initation in free radical halogenation? | Diatomic halogens are cleaved by heat or UV light, resulting in 2 free radicals. Cl2 → 2 C* |
| What happens during Propagation in free radical halogenation? | A radical produces another radical that can continue the reaction. A free radical reacts with an alkane, removing H to form HX and creating an alkyl radical. This then reacts with X2 to form an alkyl halid an X* |
| What happens during Termination in free radical halogenation? | Propagation continues until 2 free radicals combine to form a stable molecule. |
| Which substitution(primary, secondary, or tertiary) is likely to occur in a free radical bromination? | tertiary, because the tertiary radical is the most stable intermediate |
| Which substitution is likely to occur in free radical chlorination? | Chlorinations are faster and so they depend on how many Hs are present. They are more likely to replace primary hydrogens because it’s more abundant, despite its relative instability. |
| What is pyrolysis? | When a molecule is broken down by heat, also known as cracking. It’s most commonly used to reduce the average MW of heavy oils and increase the production of the more desirable volatile compounds |
| What happens in pyrolysis? | alkanes C-C bonds are split, producing alkyl radicals. These can then combine to form more alkanes. |
| What is disproportionation? | A process in which a radical transfers a hydrogen atom to another radical, producing an alkane and an alkene. |
| What is a nucleophile? | An electron-rich species that attacks postively charged or polarized atoms, known as electrophiles |
| Write out the relative nucleophilic strengths of O. | RO- > HO- > RCO2- > ROH > H2O |
| What does nucleophilic strength tend to correlate with? | basicity, if both nucleophiles have the same attacking atom, like oxygen |
| What else can determine nucleophilic ability? | size and polarability. For example, larger atoms can be better nucleophiles in a protic solvent because they can shed the protons around them and are more polarizable. However, in aprotic solvents, nucleophilicity is dependent on basicity |
| What else does a nucleophilic substitution depend on other than nucleophilicity? | The leaving group |
| What makes a good leaving group? | weak bases (conjugate bases of strong acids), or stable anions or neutral species. In other words, they can easily accommodate an electron pair; I- > Br- > Cl- > F- |
| What is a unimolecular nucleophilic substitution, or SN1 reaction? | the rate of reaction depends only on the substrate itself. The rds (rate-determining step) is the dissociation of this substrate to form a stable, positively charged carbocation |
| What are the basic steps of an SN1 reaction? | Dissociation of a molecule into a carbocation and a good leaving group, then the combination of the carbocation with a nucleophile |
| How are carbocations stabilized to help the first step of the SN1 mechanism? | Polar protic solvents with lone electron pairs help stabilize by solvating the carbocation. Also stabilized by charge delocalization, meaning more substituted cations are more stable |
| What is the rate limiting step of SN1? | The first step, creating the carbocation by dissociation of the substrate, which depends only on the concentration of the substrate |
| What can increase the rate of an SN1 reaction? | Anything that accelerates the formation of the carbocation |
| What is a bimolecular nucleophilic substitution, or SN2 reaction? | A strong nucleophile pushes its way into a compound which simultaneously displacing the leaving group, in one concerted step (which is also the rds) |
| What kind of substrates are most likely to undergo SN2 reactions? Primary, secondary, or tertiary? | Primary, so that the nucleophile will not be sterically hindered |
| What kind of transition state is formed in an SN2 reaction? | Trigonal bipyramidal, sp2 |
| Describe what happens to the stereochemistry in an SN1 reaction. | The carbocation intermediate is planar and sp2 hybridized. This means the nucleophile can attack on either side, resulting in a racemic mixture. |
| Describe what happens to the stereochemistry in an SN2 reaction. | The molecule flips, because the nucleophile attacks from the backside and the leaving group leaves from the other side. |
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schoe
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