Which of the following organic reactions does not produce a racemic mixture?
Electrophilic addition of HBr\text{HBr}HBr to propene
Electrophilic addition of HBr\text{HBr}HBr to but-1-ene
Nucleophilic addition of HCN\text{HCN}HCN to butanal
Electrophilic addition of HBr\text{HBr}HBr to trans\textit{trans}trans-hex-3-ene