Alkenes and their Bonding
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what is the difference between alkenes and alkanes
• alkanes only have single bonds between their carbon atoms - whereas alkenes have at least one double bond between a pair of carbon atoms
what does it make alkenes if they have at least one C=C bond
unsaturated
between alkanes and alkenes which one is more common
alkanes
how do you get alkenes
• they dont naturally occur in nature - so they have to be made through cracking alkanes
what would the structural formula for ethene be
CH2=CH2
what would the structural formula for propene be
CH2=CH-CH3
what would the structural formula of cis-but-2-ene be (just explain)
• there would be a C=C bond in the middle • with a CH3 coming from the left and right carbon atom • both at opposite sides from each other (Z-isomer) • two Hs would be bonded to a C each as well
what is the general formula of an alkene
CnH2n
what is a common cyclic alkene and what is its structural formula
• cyclohexene • CH=C(H)-C(H2)-C(H2)-C(H2)-C(H2)-back to the first CH • the hydrogens are in brackets as to show that it is the carbon atoms that are bonded together to make the ring
what is an advantage that alkenes have over alkanes due to their C=C bond
• they are more reactive than alkanes - so they can be used in many more useful reactions
why does using the C=C symbol seem to be misleading about how the carbon atoms are bonded
• because it implies that the two bonds between the carbon atoms are the same • when they are not
how many atoms are the carbon atoms in a C=C bond bonded to
• 2 other atoms • as two bonds are being used to bond with another carbon atoms • where it would normally have 4 • as carbon has 4 electrons in its outer shell
what does covelant bonding between atoms usually involve
• the overlapping of the orbitals of two different atoms - these could be 2 s orbitals, 2 p orbitals or one of each
what are all these bonds represented by when drawn and what is the example when drawing hydrogen gas
• a line - so hydrogen gas, H2, would be drawn as H-H
what can all of these simpler covelant bonds be called
• sigma bonds - also known as o (with a line on top) bonds
when sigma bonds are formed between carbon atoms in an alkene, what is their formation called
• formation by axial overlap - or end-on overlap
in a molecule of ethene, how many sigma bonds would each carbon atoms have
• 3 - because they are bonded with one carbon atom and 2 hydrogens each
what does that mean regarding the electrons in carbons outer shell
• the two carbon atoms each have one electron in a p orbital left • which hasnt been used in bond formation
what is the relationship between the p orbitals in this molecule of ethene and what does it mean
• they are parallel to each other • and so are not able to overlap with each other • in the same way as in the formation of sigma bonds
however, what happens when the p orbitals do overlap
it results in the formation of two regions of negative charge above the C-C sigma bond
how would this bond between p orbitals form in the first place
it is formed by the sideways overlap of orbitals
what are these types of bonds called
• pi bonds - also known as (literally the sign for pi) bonds
if each of these regions of negative charge contain one electron, what is made
• they make up a second pair of electrons • between the carbon atoms • as one region of negative charge is above the C-C sigma bond and the other is under
what is different about the electrons in the pi bond compared to the sigma bond
they are further away from the carbon atoms than the electrons in the sigma bonds are