Alkenes-organic chemistry Flashcards
Structure bonding and reactivity, addition reactions, polymers (18 cards)
What are alkenes and why are they called unsaturated hydrocarbons?
Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). They are called unsaturated because the double bond means that they have fewer hydrogen atoms than the corresponding alkanes, which are fully saturated with hydrogen.
How does bonding occur in alkenes?
In alkenes, the double covalent bond between carbon atoms consists of two parts:
Sigma bond (σ): A single bond formed by the direct overlap of atomic orbitals.
Pi bond (π): A bond formed by the sideways overlap of p orbitals, located above and below the sigma bond.
What is the significance of the double bond in alkenes?
The double bond in alkenes creates a high electron density between the carbon atoms, which makes alkenes reactive. This high electron density can attract electrophiles, making alkenes prone to reactions like electrophilic addition.
Q: What happens when alkenes react with HBr?
When alkenes react with HBr, the pi electrons of the double bond attack the H⁺ (electrophile) from HBr, forming a carbocation intermediate. The Br⁻ (nucleophile) then attacks the carbocation, forming a bromoalkane.
What happens when alkenes react with concentrated sulfuric acid (H₂SO₄)?
When alkenes react with concentrated sulfuric acid (H₂SO₄), the H⁺ ion from the acid reacts with the alkene to form a carbocation intermediate. The bisulfate ion (HSO₄⁻) then attacks the carbocation, leading to the formation of an alkyl hydrogen sulfate.
What happens when alkenes react with Br₂?
When alkenes react with Br₂, the pi electrons from the double bond attack the Br₂ molecule, leading to the formation of a bromonium ion intermediate. The Br⁻ ion then attacks the more stable carbon of the bromonium ion, resulting in the formation of a vicinal dibromide.
How can bromine be used to test for unsaturation in organic compounds?
Bromine water (Br₂ in water) reacts with unsaturated compounds like alkenes. When added to an alkene, the bromine will decolorize the solution, indicating the presence of a double bond (unsaturation).
Why do addition reactions of unsymmetrical alkenes form major and minor products?
In addition reactions of unsymmetrical alkenes, the carbocation intermediate formed during the reaction can be primary, secondary, or tertiary. The tertiary carbocation is more stable than the secondary, and the secondary is more stable than the primary. As a result, the major product is formed when the more stable carbocation is intermediate, while the minor product is formed from less stable carbocation intermediates.
How does the stability of carbocation intermediates affect the products in electrophilic addition reactions?
The stability of carbocation intermediates plays a key role in determining the product of the reaction. Tertiary carbocations are the most stable, followed by secondary, and primary carbocations are the least stable. The major product is formed by the more stable carbocation intermediate, while the minor product is formed from the less stable one.
Q: How are addition polymers formed?
Addition polymers are formed from alkenes or substituted alkenes through a process called polymerization. During this process, the double bonds in the monomer molecules break, and the monomers link together to form long-chain polymers.
What is the repeating unit of an addition polymer?
The repeating unit of an addition polymer is the structure that repeats in the polymer chain, derived from the monomer’s structure. The repeating unit includes the part of the monomer that remains after the double bond breaks and the polymerization process begins.
How are addition polymers named according to IUPAC rules?
Addition polymers are named based on the monomer used to form them. The name of the polymer is derived from the monomer name with the prefix “poly-“ added. For example, the polymer formed from ethene is poly(ethene).
Why are addition polymers unreactive?
Addition polymers are unreactive because they consist of long chains of carbon atoms with single bonds, and there are no reactive double bonds left. The absence of functional groups makes addition polymers chemically inert and resistant to many types of reactions.
What is the nature of intermolecular forces between molecules of polyalkenes?
The intermolecular forces between molecules of polyalkenes (such as poly(ethene)) are primarily van der Waals forces. These forces are relatively weak compared to covalent bonds, which explains why many addition polymers are flexible and can be molded.
How do you draw the repeating unit of an addition polymer from a monomer?
To draw the repeating unit, identify the part of the monomer that will repeat, and show the monomer’s single bond connecting it to the next unit in the polymer chain. For example, the monomer ethene (C₂H₄) will form poly(ethene) with the repeating unit:
-CH₂-CH₂-.
How do you draw the repeating unit from a section of a polymer chain?
To draw the repeating unit from a section of the polymer chain, identify the part of the chain that repeats and remove the connections to the neighboring units, leaving a single monomer unit. For example, if the polymer chain section is -CH₂-CH₂-CH₂-CH₂-, the repeating unit would be -CH₂-CH₂-.
How do you draw the structure of the monomer from a section of the polymer?
To draw the structure of the monomer from a section of the polymer, identify the repeating unit and add the necessary double bond that was present in the monomer before polymerization. For example, from the repeating unit -CH₂-CH₂-, the monomer is ethene (C₂H₄).
What are some typical uses of poly(chloroethene) (PVC), and how are its properties modified?
Poly(chloroethene), commonly known as PVC, is widely used in plumbing pipes, flooring, and electrical insulation. Its properties can be modified by adding a plasticizer, which makes the polymer more flexible and easier to process.
