Alkenes Flashcards
Alkene bonds related to alkane
Alkene bonds are shorter and higher energy. React faster as less stable. Alkenes are unsaturated while alkanes are saturated
Addition Reaction Outline
Conversion of alkene to alkane by adding more atoms (forming sigma bond) breaking pi bond in alkene and sigma bond in molecule being added. Reversible via elimination reaction
Electrophile (electrophilic) Def
Electron poor substances. Attracted to negatively charged electrons
Neutrophile (neutrophilic) Def
Electron rich substances. Attracted to positive charges eg cations
Mechanism Def
Step-by-step account of what happens during a reaction. Shows all chemical species in reaction and all bonds broken and formed
How to describe breaking/formation of bonds
Electron movement
Rate Determining (Slowest) Step of an Addition Reaction
Addition of an electrophile (breaking of pi and sigma bonds)
Carbocation
Intermediate of addition. Molecule with positively charged carbon atom (lost it’s electron). Triagonal planar geometry. 3 Types
Primary Carboation
Positive carbon is attached to 1 other C. Least stable carbocation
Secondary Carbocation
Positive C is attached to 2 other Cs. Intermediate stable carbocation
Tertiary carbocation
Positive C is attached to 3 other Cs. Most stable carbocation
What carbocation will always be formed in reaction (which pathway taken of more then 1 theoretically feesible)
Most stable
Do different carbocations produce different products
yes, different arrangement of substituents
Why do energy profile diagrams have multiple peaks
No. of transition states = no. of steps in reactions. 2nd transition state bonds go from 2D to 3D
Markinov’s Rule
When a molecule containing hydrogen is added to an unsymmetrical alkene. H+ adds to C with greatest number of Hs already attached. Secondary atom (anion) attaches to other carbon
What conditions lead to alkenes becoming alcohols
Water adding to alkene in the presence of an acid (H donating) catalyst and heat
What happens to a catalyst if it binds to a molecule in 1 step
It is regenerated in another. Catalysts don’t get used up in reactions (usually seperating 1 H from H2O)
Hydrogenation of Alkenes
Addition of H2 to alkene dissolved in suitable solvent with powdered, suspended metal catalyst. Metal catalysts break H2 bonds, alkene bonds to metal surface, H bonds to alkene. Atoms add on same side of double bond
Heterogenous Catalysis Def
Catalysts not dissolved
What does the conversion of an alkene to an alkane effect melting points
Melting point decreases
What substances experience geometric isomerism
Substituted alkenes, substituted cycloalkanes and octahedral/square planar complexes with different ligands
Geometric Isomer Def
Conversion between cis and trans isomers
Why geometric isomerisation occurs in alkenes
No free rotation around sigma ( C - C bond) because of pi bond. Break pi bond, allow rotation around sigma bond and let pi bond reform. Energy needs to be provided
Cis Isomer Def
2 identical groups on same side of double bond. Higher boiling point, lower melting point
Trans Isomer
2 different groups on same side of double bond. Lower boiling point, higher melting point
Under what conditions are they not considered geometric isomers
There must be 2 different substituents on all carbons
Medical applications of alkene isomerisation
11-cis-retinal (pentene and aldehyde) combined with opsin (protein) in rod cells in cornea. When light hits retina, rotation around sigma bond at C11 and C12 cause conversion to trans state. Protein changes shape (ospin to rhodoposin). Nerve cells stimulated and brain registers image (enabling vision)
Important Biological Alkenyl Compounds
Vitamin D3 (intestinal absorption of Ca, Fe, Mg and Zn) and Beta-Carotene (orange colour of carrots)
Alkene synthesis from alkanes
Dehydrohalogenation. Elimination. Forming separate molecules (eg water) and alkene
Dehydration of Alcohols
Alkene synthesis via elimination. Requires high conc of acidic conditions and heat. Tertiary alcohol is most reactive, primary is least
